Regardless of where you buy your balls, make sure to inspect each one before using. When buying standard balls, I've found that ~7 out of 10 have pits.

I bought the Premium Ultra Gloss balls from Pinball Life, only to find that 7 of the 10 had pits on them also. Maybe I just had bad luck, but I was pretty disappointed, considering that I bought them specifically because I was frustrated with the quality of standard balls and had heard so many people lauding the quality of the PBL Ultra Gloss.

The only balls I've found, that I'd consider excellent quality, are the SilverJets. Of course, they're more expensive and can become magnetized, which causes problems feeding from the ball trough. I believe this can be solved with the Anti-Mag Trough Shim though.