The "white stuff" is ammonium nitrate. Timothy McVeigh used it in OKC, and it is regulated a little. That is to say, the average Joe can't go buy 500lbs of it without being noticed.
Its most common use is fertilizer. Next most common, I think, is in
ANFO used in mining as an explosive.
Making your own Tannerite with ammonium nitrate is kind of a PITA. The granules have to be just the right size, you have to know exactly how much catalyst (aluminum powder) to add. Too much or too little and no boom.
All told, it truly is cheaper to buy it from Tannerite.com rather than make your own. Unless you're buying 1000lbs at a time, ammonium nitrate of the grade you make Tannerite with will be expensive.
-T.