it varies from state to state, and if you are buying off the internet it varies according to what part you are looking to buy. What state do you live in? The best is a local club or shop, that way you can get to know the guys there and get better deals and get to know whats a real performance part and what a crap part is.
since i dont know what state ur in i ll give u some of the internet sites the best one is americanmuscle.com they give free shipping and good prices and a wide variety of parts thats where i usually do my shopping when i dont take it the shop... the other one is blueoval.com they specialize in fords amd lastly stage3motorsports.com they r good but they mostly offer rousch parts. but try them out and let me know what u think
Lots of 'em, Pick up a Muscle Mustangs and Fast Fords magazine and compare prices. You can get better deals, but Summit and Jegs have been around forever, have excellent customer service, so there's less chance of you getting screwed.