I have bought from Summit, 50Resto, Brothers Performance, Trick Flow (direct), and Anderson Ford Motorsports. All have done well, and had good service.
One other thought is that you should look around for a good performance shop in your town. Then you can do a little comparing with online prices. But when it comes down to paying about 5% to 10% more to a good local shop that I like versus mail ordering, I will go with the local shop. First of all, shipping often makes up the difference, so you really are talking about the same price (especially if you have to return something!!). Second, you have lots better access to the people behind the counter than the folks on the 800 number. So you can get better advice from people who really know what they are talking about. But you gotta buy from them if you expect them to be really helpful for you.
Just a thought.