MVT is a good source for a production gun, so is Loyalist Arms (no affiliation with me btw).
Any flintlock musket or fusil does both shot and ball. I like stuff in 20 ga (.62 cal) or 28 ga. (.54 cal). You need to know that to shoot both ball and shot, they have no choke, or in modern terms are cylinder bore. So you're talking 20 yards and less for a good pattern. One way to help compensate is to use a bit larger shot. If you'd use #8 on squirrel with a modern full choked shotgun, you'll want #6 with the flinter, as you'll have less pellets on target so they will have to do more work. I like #7 1/2 on pheasants with a modified choke in my caplock, but #5 with a smooth bored flinter.
You can use paper wads as a sort of shot cup to get you about 10 more yards, giving you say 35 yard range, but you really can't get much beyond that. FORGET using plastic wads. They cause all sorts of problems.
Some guys swear by the 28 ga/.54 as they say the shot-column is longer, and gives better results, and others swear by the 20 ga. I like my 20 fine.
LD