See if you can find a ML club in the area that may have ordered in bulk for club members.
To be strictly legal (and these days, 'legal' is a pretty good thing to strictly be) an individual can't sell black powder without some Federal explosive dealer's license. Splitting an order as suggested by Roger F should be okay. If you have a place to store it, a 25lb case costs $365 from Jack's Powder Keg, delivered, and will keep you shooting for a long time.
A few years ago, I found some .440RB for sale in a shop in Pennington Gap, which indicates they may carry more than the usual fodder that inline ML shooters use. If I can remember the name of the shop I'll post it. And, have you tried the BassPro there near Pigeon Forge?
Good luck. SCL