The link you got above is a good one. Honestly, shooting these kinds of guns can lend itself to getting lots of stuff.
There are needs and nice to haves. You need to decide how much you want based upon what you're shooting (round ball bare or patched, varied shot loads) and why (range shooting vs hunting).
Dig around in the smoothbore sub-forum and you'll see a lot. Over shot cards, over powder cards, wads, patch material, screw drivercleaning jag, tow worm/patch puller, shot, roundball, spare flint and patch material will get you going (obviously you'll need to add powder). You may want a shooting bag, shot snake, breech scraper, flint wallet, etc, etc.