Endeavour tactical is the new big retailer, very nice stuff and the prices to match.
Odin tactical has been about for a while but have some good stuff.
Kit pest has just opened a bricks and mortar shop but he buys and sells high end kit.
Legit kit is us based, has their own website and via eBay.
Personally I'd avoid warrior assault kit. Its been pictured on one sas bloke about 15 years ago, it's very good copies of high end kit, it'll last ages etc but very little is an original design, most is just a heavy duty clone though. (It has been used more widely than the one bloke......but based on how was fans are you'd think that's all SF use).
There's several Facebook groups (realsteal kit and accessories, private kit trading etc)
A lot of second hand sales are now done via WhatsApp.
For clone kit, TMC has really come along way and shouldn't be ignored. shekkin gear is worth considering too. Devt team 6 have a European store and stock some good clone stuff. Semapo came and then their quality quicky went downhill. Somogear peqs are the new standard (but not used much as nods are prolific at events)
I will stress though, I did a lot of milsims for a few years (mainly with Stirling) and if you do opfor, you can get away with some cheap bdus,warm/wet gear and cheap (sensible) camping equipment. Some of the best opfor players used the same trousers and jacket at event weekend in and out, gardening gloves etc. The mindset is more important.
If you've done milsims before, know that you want high end kit and want to avoid buying twice, go for it. If you haven't done events before, do 3/4 before you spend too much. It can be done surprisingly cheaply.