it's very easy to make your own if you start with wheat gluten flour. You can purchase this in the bulk bins at whole foods or in boxed form (bob's red mill, ener-g).
this recipe is a good starting point:
and this is also delicious:
Seitan is the wheat gluten that is left after most of the starch is rinsed out and the dough is cooked. It is cooked in a number of ways; its mild taste takes on the flavor of the other things in which in which it's cooked. By kneading dough until it is firm, it is commonly used as a meat substitute and has been used by Buddhist monks for hundreds of years.
Before the carnivores start crying about "fake meat", keep in mind how long its been around. If you're expecting meat, sietan makes a lousy meat substitute - but then meat makes a lousy seitan substitute. Having been around for as long as it has it's a food in it's own right.
If you don't mind the additives it almost always in asian markets and is *much* cheaper than buying it in a natural food store - especially Whole Paycheck.