I used Diamond Source of Virginia when I began my search for diamonds, and before I found pricescope. They did a pretty good job of finding the type of stones I was searching for and the prices they quoted me were good. I opted to try to find a diamond myself because I was very particular about the specifications I wanted in my diamond. I prefer to do research and find things on my own, but they do offer a nice service if you don't have the time to try to do all of that stuff on your own.
The only thing I didn't like about them is that they only offer you a 2 day inspecton period once you receive the diamond. Most dealers that advertise here on pricescope will give you at least 7 days to inspect the diamond once you receive it. Whiteflash.com is the source I recommend to people that are looking for nice diamonds at good prices. They bring the diamond into their lab, have it inspected by a gemologist to make sure it matches the certificate, and then they ship it out to you with a free appraisal and 7 days to return it if you don't like it. It removes a lot of worries if you are a bit hesitant about purchasing a diamond from the internet.
No matter what you decide, a little time on your part can save you a lot of money when buying a diamond. Searching through pricescope will get you a great deal since all the prices are discounted.