If I want new parts I'll usually try Europarts as they're near me, a bit disorganised but not too bad.
If the price is daft and I want second hand there's a big recycler near me called ASM who are very good - once got 4 alloy wheels with almost new tyres there for £125!
For Wing mirrors I can recommend www.wingmirrorman.co.uk
And I found a very good guy down in Portsmouth who did a very rapid turnaround fixing an alternator at half the price of a reconditioned one-off the shelf
http://www.alternatorman.co.uk/ (don't think they're connected)