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you may want to check out pricescope... There are very knowledgeable people there who should certainly be able to help you find the best diamond for your price range if you tell them what size, shape, color, and clarity you are ideally hoping for and will let you know where you may have to sacrifice to get what you want. Good luck!
Some of their recommended online diamond sources are:
http://www.jamesallen.com/diamonds/advanced-diamond-search.asp
http://www.goodoldgold.com/diamondsearch.php
http://www.whiteflash.com/diamonds/default.aspx