Best places I found were here, Pistonheads, Autotrader and Ebay, especially if you sent them up to email details of any new cars that match your search criteria. I always found I would search each site every day whether I'd had an email or not and although my budget was £10k I would search price range up to £12k
Worked though. Bought mine off Ebay last weekend unseen and its far better than I could have hoped for.