Kombu itself can't really be replaced. Do NOT be tempted to use another seaweed variety in its place, as it will not have the same effect. That being said, if you can't obtain kombu for making dashi, there's a decent chance that you can get pre-made dashi stock. These instant stocks usually come in boxes filled with packages that look like large tea bags (see image). Instant dashi varies greatly in quality and flavour, but better ones generally contain fish. Oddly enough, they often do not contain kombu, partly because powdered kombu can become bitter or slimy.

Check the ingredient list to help you pick a good instant dashi. The cheapest (and most common) varieties use MSG as the primary umami-building ingredient. While they do a good job of making a rich stock, they lack the depth and characteristic 'briny' character that a properly made dashi will have.

If you are absolutely unable to get kombu or good instant dashi, dried shiitake mushrooms can be rehydrated in water to make an umami-rich stock. In fact, these mushrooms are sometimes used alongside kombu to make dashi. That being said, they do not replace the flavour of dashi, but instead provide a complimentary flavour.

JAPANESE KOMBU VARIETIES

Rest assured that using any kombu will elevate your Japanese cooking. Different varieties do impact flavour, but generally not in ways that will make or break the average dish. Nonetheless, if you have access to a particularly well-stocked Japanese grocer, try to seek out some of the different varieties below if you're looking to take your washoku to the next level.

NOTE: The Japanese names given are intended to help you shop, but take note that the transliterated names are not always given in English.  Consider printing out the Japanese characters or bringing this guide up on a mobile device to help you shop.  Note that if you do take this approach, look for the word kombu (昆布 in kanji, こんぶ in hiragana), then figure out what character(s) come before it. 

Hidaka-Kombu

Hidaka (Dashi) Kombu (Hidaka - 日高, Dashi - だし) - Probably the most commonly found kombu, Hidaka kombu is a good all-purpose variety, suitable for a wide variety of uses.  Compared to some of the other varieties, Hidaka kombu makes a relatively mild but well-balanced dashi.

Hosome Kombu (Hosome - 細目 or ほそめ) - A young kombu, harvested in its first year of growth, this variety has a very slippery texture when cooked.  It is not usually sold or used for making dashi, but is a preferred variety for making the condiment known as tororo kombu.  It has the whitest edges of any kombu and thin leaves.

Ma-Kombu

Ma Kombu (Ma - 真 or ま) - A very popular, glutamate-rich kombu for making full-bodied dashi stock.  While it has an excellent taste, it can sometimes produce a cloudy broth, so it is generally not favoured for making clear soups.  That being said, the flavour is considered to be so good that some chefs will use it for everything.  Ma kombu benefits from the cold water soaking technique described above.

Naga Kombu (Naga - 長 or なが) - Naga means long, or lengthy, so it shouldn't come as any surprise that this variety of kelp routinely grows 10-20 m long.  The flavour profile of this variety can be somewhat eggy, making it less popular for soups and stocks, but very well suited to preparing nimono (simmered vegetables).  Naga kombu is usually sold in long strips that are much thinner than most of the other kombu varieties (though Hidaka kombu can look similar in some cases).  

Saomae Kombu (Saomae - 棹前 or さおまえ) is an expensive variation on naga kombu harvested in the summer of its first year of development. The kombu pictured on the right side of the featured photo above is Naga kombu. 

Rausu Kombu (Rausu - 羅臼 or らうす) - A favorite for vegetarian cooking thanks to its rich, meaty flavour.  Rausu kombu is also very popular for making fish and for eating on its own.  It is divided into two types based on color; black kurokuchi and reddish-brown akakuchi. 

Rishiri-Kombu

Rishiri Kombu (Rishiri - 利尻 or りしり) - Sweeter, saltier, and more herbal than ma kombu, this variety is known for making crystal clear stocks that are ideal for clear soups.  Rishiri kombu benefits greatly from the cold water soaking technique described above, and can even be soaked in cold water for days at a time (in the refrigerator) to enhance its flavour. You may find some packaging or resources that refer to the various kombu types by latin names.  Currently, the accepted scientific status has it that all of the varieties are part of the species Saccharina japonica, formerly Laminaria japonica.  Rishiri kombu has been called a different species before (Laminaria ochotenis), but is currently classified as a variety or subspecies of S. japonica. 

OTHER KOMBU USES

Many other products are made from kombu, but these are generally not used for making dashi.  As a general rule, if your kombu is not being sold in large, intact sheets/leaves, don't use it for making dashi!  Nonetheless, here's a basic primer on some of the kombu preparations. 

• Kizami kombu, musubi kombu, and oshaburi kombu are all sliced preparations of kombu used as snacks or ingredients in cooked dishes.  
• Shio kombu and tsukudani kombu are both kombu products that have been simmered with a variety of ingredients, and are designed to be eaten on their own or in a dish.  
• Tororo kombu is pressed, shaved, marinated kombu used in soups or with rice dishes.  When tororo kombu is shredded, it becomes furikake tororo, which is sprinkled on rice.
• A high quality (and expensive) variety of kombu called shiroita kombu is made by planing the outer surface of the ma or rishiri kombu leaf and then marinating the shavings in spirit vinegar.  Shiroita kombu is usually used to prepare and present certain fish dishes.

IODINE & HEALTH

High-iodine foods like kombu can have both positive and negative impacts on our health.

Iodine is extremely important for the function of the thyroid gland, and insufficient iodine intake can lead to thyroid dysfunction, neurological problems, and more.  However, excessive intake of iodine can lead to hyperthyroidism (thyroid overactivity).

Kombu (and seaweed in general) is very high in iodine.  The element is extracted from seawater and accumulated it in high quantities in the tissues.  The actual quantity of iodine varies enormously between seaweed species, and depending on cooking and other preparation methods.  Boiling kombu, for example, extracts much of the iodine from the seaweed, and transfers it to the stock.  Because of this, dashi is generally quite high in iodine, while the leftover kombu itself may actually contain very little.

It would be impossible (not to mention medically irresponsible) for me to make a sweeping generalization about eating seaweed and monitoring your iodine content.  It is safe to say that if kombu (or any other seaweed) is only an occasional component of your diet that you're unlikely to have a major impact on your overall iodine levels one way or another.  If you are at risk of suffering low iodine levels, adding seaweed to your diet can be a good idea - though it can be difficult to assess whether or not this is needed, and what quantities of seaweed would ultimately be needed to make up for any dietary insufficiency.  Iodine deficiency has become rare in much of the developed and developing world, though there are still some risks for certain populations.  Pregnant women, those who avoid iodized salt, and vegans are at an increased risk for iodine deficiency (unless they're already eating plenty of seaweed, that is).  Over-consumption of iodine is a fairly low risk for most people, though it would generally be unwise to consume extremely large quantities of seaweed on a daily basis.

If you have any reason to be concerned about your iodine levels, thyroid function, or how kombu (or other seaweeds) can safely fit in your diet, consult a physician.