Beware of any site that offers equipment well below the suggested price. Often it is gray market equipment which is sold outside the US and does not carry a warranty. Other stories I have heard include equipment not being as advertised. An example of this would be a lens for a Canon camera being advertised and when it arrives, it is a knock off brand of poor quality.
I have purchased equipment from B&H Photo based in NY City. Their prices are reasonable, shipping is fast, and selection is excellent. I have read lots of books written by pro photographers and they often recommend B&H.
Ritz Camera has a decent selection and carries a good choice of store brand merchandise (made by various name brand manufacturers). Other stores/sites I have dealt with include Calumet in Boston, Adorama in NY, and Hunts Photo and Video.