Any advice on this ? I would like to use them, but i just cant see how. 
It's probably easier to comprehend for people who were buying before the internet took online shopping to a new level. PBR is the old school retail site, not a real e-commerce site.
PBR specialise in Gottlieb, but do have a wide range of other parts.
You have to locate your part number in a manual and when you have a list of wanted stuff, email them for availability.
While their web site is not a genuine e-commerce system, if you know your way around your games, you can use the "Game Specific" sections for Gottlieb parts as they standard parts in say a HH as the same as used in System 80 games for the years around that game.
I am amazed how often this issue is discussed, but I think we've all come to expect genuine e-commece sites these days, so much so that PBR seems an anachronism.
