i've had okay luck with http://www.bearingballstore.com/ (chrome and carbon steel balls in wholesale quantities).
Only thing i'd say is to do a quick inspect before they go into a game (i've had a few that had defects which i, simply, tossed).
My thing has lately has been not giving a damn about durability and replacing them all en masse every 150 games or so. I've not found any brand to be be MARKEDLY better or worse in terms of taking nicks from regular use (given that ball damage is a rather random process to begin with). As long as the surface is right at install - play em and pitch em when they get shitty.
When i started out - i was buying silverjets and have since stopped that - opting for a more aggressive replacement regimen.