Most vendors buy from the same few manufacturers, and Scott Drake makes most of the common repop parts, so I've found that there isn't much variation in parts quality except when it comes to sheet metal. However you need to stick with somebody who's advice you can trust. I've used Mustangs Unlimited, CJ Pony, and Mustangs Plus, all with good results. I like the Mustangs Plus catalogue, since it contains some helpful repair articles and tips.