I never need to replace them, but from time to time I just re buff them myself. 6" cotton wheel and a little bit of grey then white bar polish and they look great and the surface is sealed to keep in the free iron, something that a tumbler doesn't do.
It's actually quite easy and no, not dangerous at all if you're smart and careful. I just roll them in my fingers against the wheel for a short while until the entire surface is done.
I've owned a metal polishing shop for 35+ years and with 70+ machines, that's what works for me.