Welcome from Missouri!
My 1851 would jam up when I'd try to thumb back the hammer.... I would have to let the hammer back down and thumb it back a second time almost every time. All I had to do was adjust the hammer spring under the grip a little bit. Now it works as it should.
I get my powder from Graf and sons out of Mexico, Missouri. There's other vendors that normally stock it, but I've had no trouble getting it from Graf. They have an "email me when back in stock" option.
Good luck, shoot straight and God bless,