My bore slugs at .511. I haven't bought bullets for it, but if you decide to start casting your own, I have a mold recommendation. Also, I strongly advise against Lyman reloading dies and suggest RCBS as giving more satisfactory results.
These topics have been discussed at length in these pages and there may be more complete answers there. At any rate, my reproduction Spencer is one of my favorite guns to shoot. It certainly has its quirks, but I you'll have fun with it!
CC Griff