I have had my best luck with NPD. They have good prices and great customer service. If you aren't satisfied with any of their parts you can return them easily. I have done this a couple of times and they are always easy to deal with.
I would recomend Matt at Pondarosa Mustang Not the biggest but the best customer service you will find. When you call your talking to the boss. ALways better to call than email. Has been long time surporter of this site.
I have had nothing but greatness from NPD. No problems from Mustang Depot, but they lean more towards Restomod supplies and can be backordered sometimes. Stay away from The Paddock, there prices are great and so is there costomer service (my expereinces, some people have had crapy service) but the quality of parts (especially sheet metal) is a crap shoot sometimes it's goos but sometimes its horible. They always seem to have the the parts you need out of stock, and back order will range from two weeks to months .Just don't try to find out yourself. California Mustang has also been very good to me.
Mustang Depot ++
California Mustang ++
The Paddock ---
though those are only MY opinions and experiences.
I just had a horrible experience with Mustangs Unlimited. After receiving a very large order I discovered that half of the items were not in the boxes even though the invoice said they had been shipped. I spent over an hour on the phone with them trying to resolve the situation. All said and done it took me 22 days to get all of my parts. I would go with NPD if I was you.
I like Mustang Unlimited and NPD. I would only suggest that you don't use the Paddock. I live in Indiana only 45 minutes from the Paddock and don't use them. Everytime I tried in the past, half of my items were on back order and the ones they had in stock the quality was not what I would expect. I returned many items to the Paddock due to they did not fit or the quality was not good. This occured multiple times over the past several years. They refuse to tell you who makes the part you are ordering and the staff knows nothing about Mustangs. They sell the lowest quality products on the market. I even ordered parts from Mustang Unlimited on the east coast and the Paddock on the same day and got the parts from Mustangs Unlimited several days before I recevied them from the Paddock that is only 45 minutes away!
VA Mustang is pretty good, but their prices tend to be a tad higher for comparable parts. I've asked them about this and was told that they process ~100 orders a day and aren't trying to have the best prices. The key is to use a reliable supplier that's close enough to reduce shipping, but in another state so that sales tax is not charged.