You might want to try Shooters Choice (the gun room) in Waterloo.
They carry quite a variety of black powder stuff and they are my personal source for real black powder.
I only get there once or twice a year and can't remember for sure what all they stock. But I have got round balls from them for both my 40 cal and 38 cal front stuffers.
Google them and drop them a note - not much on their website that I recall, but will answer your questions in short order - good to deal with.