Ebay tends to have a lot of junk if you're trawling for lowest prices
- try police sales of recovered bikes, also ask at proper bike shops
for stuff they need to shift (but don't want to put on the shop floor)

Unlike most London shops - the London bike shops do still tend to be
run by enthusiasts - go chat to them.

The one in Tooley St (On yer bike?) is good, as is the one in Grays
Inn Road (Condor Cycles?), and the one down the side of Waterloo
(Evans?). They sell expensive bikes - but I would be surprised if they
wouldn't point you in the right direction.

Also try London Cycling Campaign.