Used camera specialists
mbp a lot of good photography suppliers like
wex do part ex and sell the used stuff with warranty. I'd be careful with fleabay though.
avforums is a good place to go as well as advice there is also a classified section for cameras etc there's a Nikon D90 + 18-105 + 70-300 on there at the moment for £300.
What I would suggest before buying is going somewhere and having a play as things can be very different when you have it in your hand, the ergonomics the menus/dials/controls may or may not suit. I personally didn't like the Nikon controls and menus, the canon, to me, was much better.