I buy from a variety of sources depending on my needs. (Heck, I even found a reliable supplier for genuine Bosch MAF assemblies on ebay, though I'm usually loathe to buy anything for a benz off ebay.)

One thing I like about Pelican Parts is they usually offer a few different brands and have competitive prices.

Plus, so far as I've seen, they don't sell URO. That means I can recommend them to others without any concern.