while you don't say where you are, look to US based sources as a start. Photographers Formulary, and Bostick and Sullivan.

Amm Dichomate though the mail internationally may be a hassle; I think it can be an explosive device precursor if you intentionally abuse it enough, in the right manner.

Then again, rice left to go moldy, I believe can become a bioterror issue. But I can still buy that 20lbs or more at a time at the grocery store. I guess the great war on terror has not turned every potential hazard stone yet, or more tongue in cheek, perhasp caught a bit of common sense in its stone turning to date.