The first item you should purchase is a good book on the subject... it will save you much grief later.
Pedersoli's Harpers Ferry is listed as .58 Caliber, so you will need the appropriate round ball and patches... patched can be purchased dry or pre lubed, pre lubed patches will save you from lubing them. Bore butter or wonderlube will keep fouling soft and easy to clean, and can be used as a preservative after you have cleaned the pistol.
You will need a leather or lead jaw pad and the correct size flint (along with spares... once you find decent flints, you might want to lay in a good supply.). You will want a properly sized screwdriver to install the flint. A .58 caliber cleaning kit, a vent pick and brush, a .58 caliber wiper, breech plug scraper and ball puller are mandatory.
You might want to get a CO2 powered ball discharger to dislodge an unfired ball... they do work better on percussion fired firearms, but it is worth a shot.
Just a quick and dirty list for starters.