I'd have a look around ebay first (just to get an idea of prices) then head to my local army surplus. I got a set of desert dpm's, that was trousers, ubacs (you'll find it hard getting one of these in dpm cheap), and smock for under £30 at my local and another £20 for a G1 pair of desert boots. DPM stuff and olive green can be found very cheap
As a 12yo I would recommend steering clear of anything expensive, you are likely to grow out of them very quickly so cheap and cheerful is probably the best bet. Having stuff being a little baggy now will give you room to grow into it without looking out of place as kit tends to be worn a little baggy. Don't go nuts though you don't want everything dragging through the mud.
Alternatively, you may be getting close to the age to join a cadet force, they issue kit and will swap it when you need bigger sizes. You might find they aren't happy with people using the kit outside of their activities so might be worth looking into that first.
Down side is you have to turn up to their evenings, learn to march etc, up side is you will get to learn about real guns and learn a whole bunch of new skills. I learned to glide in my air cadet days.
