I can't STAND Bavauto.

Auto Parts at AutohausAZ - OEM Auto Parts - Discount Replacement Parts, Resources and Car Care Tips is my first go-to.

Pelican Parts has a good selection, but often at a higher price.

OEM / Performance Parts for Audi, BMW, Mercedes Benz, Mini, Porsche & Volkswagen - ECS Tuning has clear distinction between OEM and OE parts, often along with great pics for reference, too. Not the best on price.

getBMWparts.com is my favorite OEM supplier when all else fails.