jrkrk The only way you can reliably use Pyrodex in a flintlock is to duplex the load. This means that you must have a small amount of blackpowder dumped into the barrel near the breech, the remainder of the charge can be pyrodex. As pendennis stated very correctly, flintlock ignition isn't hot enough to reliably ignite pyrodex powder.
If you want to shoot flintlocks but can't find a shop local to buy powder, my suggestion would be to look ONLINE! There are several master distributors who will ship powder straight to your door, depending on the type of powder you want. James Worthington, owner of Maine Powder House has Elephant, Schuetzen and Swiss. Graf & Sons (Correct me if I'm wrong gentlemen) stocks Goex. Maine Powder House sells his powder in 25 lb. cases. I am pretty sure you can buy as little as 5 lbs. from Graf & Sons. The reason I don't know much about them is I don't use Goex and haven't ordered from them.
I primarily use Elephant and Swiss and for the past 5-6 years we've been buying all our powder from James. His site can be found at
www.mainepowderhouse.com I don't know about the other place.
In any event, you could order your powder and then try the flintlock out. You can also use 3fg instad of 4fg for priming charge.