Goex, Swiss, KIK, Wano, Graf, Elephant (which you could not sell me), and others are all good black powders. Swiss needs to be watched close as it is much stronger then Goex and the others. 15% stronger in most cases.
As for substitute powders, while you can duplex many powders out there and get them to fire, black powder is still the best thing for a flintlock.
I purchase my black powder by the case through Powder Inc. I get a good deal and even though there is a hazmat fee, it all works out to make Goex the most cost effective powder out there.
Granted, I pick up maybe 10 pounds of Pyrodex and Triple Se7en at Wal Mart's end of the season sale, but I do not count on that supply for my shooting needs.