If it's a .75 caliber and you want to shoot shot, I would start with 70 to 80 grains of FFg, an 11 gauge thick card over powder wad, maybe a fiber cushion wad, and then 1 1/8 to 1 1/4 ounce of shot followed by a thin card overshot wad to hold the load in the barrel until you shoot.
An old rule of thumb is to use the same measuring scoop for both the powder and shot.
You can get lead shot from places that sell shotgun reloading supplies or order it from MidwayUSA. It normally comes in 25 pound bags. I have never had problems getting it locally where I live.
Track of the Wolf has card wads in every concievable gauge. 11 should be right for .75 caliber. If it's .66, 16 gauge wads should work.
A musket has an unchoked barrel so expect cylinder choke patterns and range. Experimenting with different powder charges may give a tighter pattern. Using modern plastic shot protector wads will probably tighten up the pattern but you'll still want a card wad over the powder because a 12 gauge shot protector will be a little too loose in a .75 caliber barrel.