Aside from the jacket and trousers, the majority of everything else that you need can be purchased off the shelf. For example, the vests and holsters are from Blackhawk Industries, the BDU belt, pistol belt, boots, are also items you can buy.
Which season/character are you looking to recreate? They changed uniforms quite a few times. Luckily though the tactical gear has remained mostly the same except for a variant here and there.
For weapons, there's a couple of tutorials on building the P90s on my site SG1Props.com