Many supermarkets have it in the specialty flours section, which is usually right next to where you would find the usual all-purpose and bread flours.

Check health food stores. Since it's only 1 T, you could leave it out -- it helps support the rising and gluten formation with a shot of protein.

I've seen it at Whole Foods and at natural foods stores that sell baking ingredients in bulk. Also, other grocery stores that sell Bobs Red Mill products often sell it. I believe it's called "Vital Gluten." I strongly recommend that you try to find it in bulk, however, as you only need a tiny bit and the packages are large, relative to your likely usage in the future. And if you can't get it, don't worry, really. You can make this bread without it. The crumb might be a bit more dense, but it will be just as tasty. Just add an equal amount of bread flour. (Or, send me your address via the private messaging system through my cook's profile, and I'll pop a few tablespoons in a small padded envelope via US mail!!) ;o)

You do not need to buy it. We use to make it all the time by making dough (flour and water only) and knead a bit. Then under slow running water, wash away the starch while knead the dough in your hands. You will be left with a big clump of gluten.