Local gun shops face some pretty daunting issues when dealing with black powder.
1. There are the hazmat fees as it has to be shipped as an explosive, rather than as a flammable solid like smokeless powder, so ordering small quantities incurs significant shipping costs.
2. It has to be stored in a powder magazine and some local fire codes can be a serious impediment. For example, the gun shop in Ayden NC no longer sells black powder as they local fire code required it to be stores in a steel powder magazine with blow out panel that had to be painted red and left within 6 ft of the front door at night. Presumably so they could see it and keep a hose on it in the event of a fire. That meant they had to wheel the powder safe out front every evening and wheel it back every morning.
3. City ordinances also often limit the amount that can be stored in any one place, and that can cause issues with shops as while you can ship 50 pounds at a time per DOT rules, you can't always store that much per some city ordinances, so that causes issues for guns shops that either have to accept higher hazmat costs for smaller, more frequent shipments, or they have to split the shipment between multiple locations.
4. Insurance companies usually pitch a fit and want to charge an extra premium because you're storing an "explosive" in the building.
That more or less means that it's pretty rare to find a local gun shop that still sells black powder.
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I normally order from Powder Inc. They sell Goex, Swiss and Schetzen so they can accommodate a wide range of tastes. Last time I ordered they would ship in 25 or 50 pound quantities, but they'l mix and match the powders in your shipment as long as the containers are compatible, so you can split the order between Fg, FFg, FFFg, etc. in what ever sub quantities you need.
Buffalo Arms also ships black powder in quantities from 5 to 50 pounds, and has similar bulk pricing levels. The fixed hazmat fee however starts to get steep with a small powder quantity.
I normally get 50 pounds at a time to minimize the per pound cost of the hazmat fee. Between the bulk pricing and splitting the hazmat fee over the entire shipment, the cost of powder is down around $15 per pound delivered to your door. The downside of course is that you need to save up a fair chunk of change to buy 50 pounds at a time. Once you do that the first time though just throw $15 in a coffee can every time you pull a new pound out of the magazine and then just re-order when you get down to the last pound. You can also split it 2-3 ways on your end if you've got a few BP shooters in the area.