The best price on genuine Porsche stuff is most likely Suncoast Porsche in Sarasota, Florida. Their prices are quite low for a dealer.
http://www.e-partssales.com/Merchant2/merchant.mv?Screen=CTGY&Store_Code=P&Category_Code=PO
Several mail order catalog companies exist, such as Tweeks, Performance Products, and Vertex. All of these are on the WEb. They have occasionally good prices, but tend to be a bit high. they have some OEM stuff along with 2nd party stuff.
Another good way to find stuff is through Porswche club of America. You get PCA's monthly magazine which has many advertisements, ranging from P-dealers to custom-tweaking shops.