Not sure what "in bulk" means to you.
However, ATF limits the annual purchase per person at 50 lbs per year I believe.
I buy a case at a time, which is 25, one pound cans.
There are a several places to get your powder. I have been purchasing from Crater Fireworks out of IL for several years, it is easy for me as I pick it up on my way to or from when traveling. Buffalo Arms has a good price on powder.
Fleener