That's what I do. It only takes a very small amount. I usually plant a block of brassicas approximately 100 feet long and five feet wide. To dust that block, I use maybe half a cup of Btk dust, and I mix it with a cup of flour as an extender just to help me get it spread evenly throughout the patch. I prefer to do this on a windy day, walking against the wind if possible, because the wind helps it spread around the garden. Make a shaker with a quart jar and canning lid, poking maybe 6 or 8 fine holes in it with a nail or ice pick. When dusting, hold the jar high above the plants, and let it disperse a lot. It's not like a traditional dust that kills with contact and ingestion in a certain dose, this is biological warfare, so a few spores will do the trick.
Btk dust should be available at almost any garden center, hardware, or general merchandise store, and it's pretty cheap, a $6 canister lasts me most of the season.
http://www.google.com/#q=bacillus+thuringiensis+dust&hl=en&prmd=imvns&source=lnms&tbm=shop&ei=gsnGT5HJJeLw0gGCrPSjCw&sa=X&oi=mode_link&ct=mode&cd=6&ved=0CEoQ_AUoBQ&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=17bec49d44d7d057&biw=1366&bih=677