this was posted else where but thought it was worth showing the quality of the balls supplied with a NIB, The silverjets are available from Macro but RTBB supply a few different types of good quality balls to suit a machine with and without magnets

Pin Balls
as the balls issued with the Stern machines are known to not be that great, so replace straight away
http://www.rtbb.com.au/catalog/index.php?cPath=140

check with Greg as the size you need to get is a 1 1/16" ball