www.ipdb.org International Pinball Database.  Great place to look up game ratings and information.

www.pinballlife.com Cheapest place to order the most common parts out there. 

www.bayareaamusements.com Have a lot of game specific parts, a lot of them hard to find.

www.marcospecialties.com Like Bay Area Amusements, they carry a lot of hard to find parts.

www.pinballnews.com You can get the latest pinball news plus reviews of the newest games here.