New or used?
If you are in the US or Canada then I don't think you can beat B&H for new equipment. Ground shipping is often free. In Canada, it's so much cheaper then the cheapest Canadian equipment distributor that it is worth it to have have things shipped to your door and pay for the crazy brokerage prices and the tax. I am lucky enough to live close the border so I have items shipped to a border store on the US side then drive down, pick it up and only pay the tax when crossing to Canada.
I bought things from Adorama as well but I find B&H slightly cheaper and the service is amazing.
For used items you could use eBay or some of the ebay offshoots. Kijiji (owned by eBay) is very popular in Canada. It's a nation wide classified adds service that is very easy to use and defaults to local adds. I sold all of my old camera equipment (and 2 cars) on Kijiji and purchased some as well although I only buy brand new equipment when it comes to cameras and lenses.