Minies in several diameters are sometimes sold on e-bay, as are used moulds.
Go to "sporting goods" and search "mini bullet" "lead bullets" "blackpowder" "mould" "mini mould" etc.
Some folks selling on e-bay also have "e-bay stores" to sell their products. If you don't see the diameter you are looking for, click on "visit my e-bay store" or something like that, and browse to see if they offer different diameters of bullets.
Most folks who shoot minis prefer bore size bullets for best accuracy. Smaller daimeter bullets might work better is you plan on using paper cartridges.
Decide what you want and go for it.
J.D.