In my experience these are equal to or better than the Silverjet balls for much much less..
http://www.niftyled.com/cascade-premium-pinballs/
You get 5 balls for $16.95 from Marco or 7 Balls from Nifty for 15.99
Just like Silverjet, these are NOT made for games with Magnets
For standard balls http://www.niftyled.com/standard-arcade-pinballs/