You do realize that 2013-2014 season starts after Christmas and runs for a couple of weeks?
Pa allows .44cal and larger for "big game" which includes deer, bear, elk, and turkey. .40cal and smaller for small game!
.50cal supplies are more commonly found than other calibers. That said a .45 RB weighs ~125gr, .50 ~175gr, .54 ~230gr, .58 ~270gr, and .62 ~330gr. RBs shed their velocity quickly, so figure on giving up literally half of your MV by 100 yards. The larger calibers have an advantage in that they retain more, but require more powder and lead.
I have one of each, but if choosing only between a .45 and .50, I would choose the .50. Neither makes much recoil with hunting loads, both can be loaded down for target/short range.
For a flintlock you WILL need REAL BP, and that isn't found in every sporting goods store.