I would basically home in on the brands that interest you and go from there.
Brands like Kombat, Viper and NUPROL are mass market, easy to find and whilst good enough to skirmish in, I wouldn't trust to last long.
Surplus gear is going to be pretty hard wearing and US stuff is often made by some high-end brands, like Eagle Industries, Propper, Patagonia, Orc Industries and Tru Spec. Looking for these brands will throw up an array of retailers and you can look for deals. UK surplus is usually pretty cheap, and MTP and DPM gear will be easy to source too.
If you want high end, hard wearing gear, then 5.11, Crye Precision, TAD Gear, Arcteryx and Beyond are brands I would focus on and eBay can be a source of relatively expensive gear for affordable prices. There is a lot of what may be UKSF CP gear on there now for Tru Spec and Propper prices.