Good kitchen stores tend to carry it, and you can also order it from King Arthur.

It's a pretty broad term can mean many things.
http://www.breadmachinedigest.com/tips/dough-enhancers-and-how-to-use-them.php

I'm thinking your speaking of "Wheat Gluten" as that is the most common; they should have it at WFs. Just ask.
Most supermarkets have it in the baking section, along with health food stores.