Casting ball is the only smart way to go, unless you have a large budget and can access a store or the Internet to get exactly what you want.
I spent about two hours casting 60 bullets for my wife's 54, but my success rate is about 45%, and I am still learning how to cast bullets. I would estimate I could cast some 200+ balls for my 62 in that time.
I have never seen a non-military muzzleloader that shot musket caps. And I have seen more than a couple.
Powder has turned out to be the biggest issue for me, as I am a tightwad and I am running out of $6 per pound 3F. And my distributor buddy passed away some time back. So I will have to go retail. And I only have about 100# of lead, so will have to start hunting lead soon, too. Don't let the issue of ball or powder sway you from what you want; however, I DO plan to get a 40 for me and one for my wife, so we can deer and squirrel hunt on the cheap (and my shoulder don't hurt as much behind a 40 as a 62).