Agree. Try OPC first. There are some egregious marks up floating around. As an example- GT3 brake ducts for a 997/987 regularly advertised online for c.£30. They're £5.23 a pair from Porsche!
Bought quite a few bits, apart from the previously mentioned suppliers, from FVD Brombacher. You DO need to shop around because it all depends on what you are buying.