I always use Tischer as a price reference. Generally, I look for preowned OEM parts on the forum and I use Tischer as a reference for what the new prices are. Examples would be people selling OEM M performance exhausts used for like $100 cheaper than Tischer's prices...lol.
I'm in no way a Tischer fanboy, I always cross shop Tischer's price with other vendors and local dealerships. 9/10 times, Tischer always comes out the cheapest, even with shipping or pick up costs. If you're willing to put the effort in, generally you'll find the best prices. Comparable forum dealers are also Mckenna and United BMW...but you have to contact the forum reps instead of parts department directly.
For Tischer, if you can't find the part you need PM him:
http://www.m3post.com/forums/member.php?u=25320
Jason logs on at least once a day and is responsive.