Ffordes, MPB, Mifsuds, Harrisons Cameras, Aperture, Park Cameras, WEX and Fixation are all reputable online dealers with bricks and mortar shops, and all give a warranty on used gear, 12 months in the case of Harrisons. You may find what you want on eBay, indeed I bought my Canon 1D MkIII there, but as ever you really need to do your homework on the seller.