The National Muzzle Loading Rifle Association has regular meets that include powder vendors. At least the south western meet here in Phoenix has a powder vendor.

http://www.nmlra.org/

Check through the various tabs to see if they have something of interest for you:

http://www.nmlra.org/shoots.asp?Cat=NMLRA%20Sponsored

If powder is your thing I would call ahead of time to see if there will be a powder vendor on site. As an FYI the powder vendor here has a minimum purchase amount. I do not remember the exact amount maybe 5 cans or so.

Good luck.