Another option you have is to buy stock balls and throw them in a tumbler or vibrating brass cleaner.  I do this with all my used pinballs about once a year and they come out looking like the mirror glazed balls do.  You generally have to leave them in the tumbler for about 48 hours.  I use walnut media saturated in ruby red.  After you are done just throw out any balls that have deep scratches and buy new ones.  In the long run you save money and you have to do this to mirror glazed balls after a bit anyway.

we do this to all the Pinballs here. every pinball that comes in goes in for a few days
then we even buff them to bring up the shine more