Surplus stuff would be a good idea. Might not be original or offer the best concealment depending on what your site is like, but you can pick up an MTP UBACS for £20 and a pair of trousers for £18.
Bullseye Country Sports & Baseline supplies sell Invader Gear clothes which look good quality and they do lots of camo such as CADPAT, MARPAT, A-TACS FG (isn't genuine pattern but close!)
Military1st and Flecktarn.co.uk are good websites for gear and clothing too. I also really like the tactical gear websites (Tactical Kit, UK Tactical)
UK Tactical always has a load of stuff on sale really cheap, whereas Tactical Kit doesn't have as many sales but they do a load of giveaways on their FB page.
I would highly recommend UK Tactical for pouches. For a chest rig or vest you have a lot of choice so any of the websites I've mentioned. Tactical Kit for eye pro and things.
Asian airsoft websites for misc stuff, accessories and so on (I'll list some later when I'm on my computer) like a FAST helmet. Hope this helps!