Go to the GOEX website, which has a map of the US and the names, addresses, phone, and email addresses for their distributors around the country. Find the one(s) near you, and contact them. They can tell you the dealers who sell their products, and where they are located.
Federal law prohibits dealers from displaying Black Powder on the floor. If they have it, it has to be stored in a safe in a back room( or other building). You have to ASK Retailers if they have it-- they aren't smart enough to put up signs that tell you they do have black powder!-- Or display Empty powder cans out on the floor, with a sign instructing you to ASK for the powder.
We have lists of suppliers on this forum- see LINKS- up under member resources-- that will ship the powder to your door. If you buy in volume, you save money on the cost per pound. You can order up to 2 cases per shipment ( 50 lbs.) to spread the Haz Mat fee over more powder, and thereby cut the cost of the powder per lb.
Buying one can- 1 lb.-- of powder at a time is just plain silly, even when it was available in local stores. When I get as low as 3 lbs. I begin making plans to buy more powder!
Black Powder doesn't spoil( like modern smokeless powder can), or lose its power over time. The substitute powder do, once the can is opened. Black Powder recovered from hundreds-year-old-sunken cannons, and dried, has been found to not only ignite, but have just as much power as any new powder has.
So, save the sheckles, get with other shooters, and combine a powder order that will buy the powder at the lowest cost you can get. Keep a supply of powder to feed that gun for as long as you can afford to shoot.
Guys who shoot weekly often can go through a case of powder (25 lbs.) a year! Those who shoot competitively, and shoot more often, will go through a couple of cases of powder a year, and then some. Shotgunners tend to consume the most powder, shooting clay target sports.