I have a variety of places that I use.
Steve White Motors (forum vendor) has been one of choice for OEM parts at a discounted price, Great service and prices.
I also use Amazon and ebay. It depends a lot on what I am buying. I generally get OEM mechanical parts from SW, tires from Tire Rack, filters from Amazon and trim parts from ebay.
I also use my local dealer when I need stuff fast (when I need an immediate repair).
For custom stuff, it's a mix of everything.