http://www.jsaccessories.co.uk/index.html
Some pretty good bargains to be had there. If you have a J&S store nearby its also worth going for a browse in the actual store. Each one usually has store specific offers on that dont get advertised. Last years stock/colours etc.
Also try having a look at your local Hein Gericke shop. Top quality kit there at pretty good prices.
However, my first port of call would be good auld eBay.
Depending on yer luck with individual auctions, you might grab a bargain.
I managed to snag a 1 piece Dainese K series suit last year retail price @£800, for a measly £170. Minted condition and hardly used:-
Only problem with buying on eBay is the size and fit. If you are looking at a particular set or make on eBay, its an idea to visit a shop before commiting to buy and trying on an identical set, like Freefaller said.
The Dainese set I got is quite a tight fit, especially with the back protector underneath, but I like it that way rather than them being too baggy. All Italian leathers seem to err on the smaller size compared to whats on the label. Mine are continental "52" sized, which equates to around a 44" chest and 34" waist.