Here's some favs, in no particular order:

http://www.partsplaceinc.com/
http://www.blauparts.com/
http://www.rapidparts.com/
http://www.germanautoparts.com/
http://www.autotech.com/
http://www.eurosportacc.com/
http://www.gprparts.com/shop/
http://www.techtonicstuning.com/
http://www.vwdieselparts.com/
http://www.ecstuning.com/
http://www.shokan.com/