I've found it's also called "bread improver" or "yeast food". In my mind, it's a powder like substance.
King Arthur flour has bread improvers, here is a link:
I have found soy lecithin labeled as an "enhancer" for baked goods, though when I used it, I didn't notice any difference. Any recipe I have ever seen that calls for it also makes it optional, so I haven't bothered to look into the issue any further. But besides a specialty baking store, you best bet is to turn to the Internet. Amazon anyone??