Per Federal BATF regs, you're allowed to have and store up to 50lbs in a single family dwelling without any special permit or storage requirements.....even the Patriot Act didn't change anything regarding the recreational use of BP.
Note: I think a couple of states like NY and NJ have imposed their own additional laws limiting the amount to 5lbs or something...each state's "Fire Marshall's Office" is the official contact point about it.
http://www.goexpowder.com/distributors.html
Over time, I've bought 4 cases using J&JPyrotechnic in Moscow, PA.