Per Valorum, in the comments:
The uniforms appear to be not surplus, but US military issue that was
given to studio for their use. "Working with a staff of ten,
McQuarrie sourced her clothes from Vancouver and also New York and Los
Angeles. She worked closely with a U.S. Air Force representative in
designing the garb of the SG-1 team. In fact, the group's
fully-dressed field uniforms are authentic military issue." -
For more information, there is a Stargate costume resource (fan-made) which has sought to identify each of the specific props used. Obviously, there were some cutom-made bits (like the SG-unit patches) but there are fan-made versions of those available.
Most of the typical uniform pieces (boots, jackets, pants, vests, etc) are sourced from the same places the US military sources them, and are available for civilian purchase as well.
For instance, the SG-3 and SG-5 Marine off-world uniform consisted of:
- USMC "Hot" or "Temperate" boots OR Black Magnum "Response" (formerly known as "Trooper") OR Magnum "Stealth" boots.
- Woodland BDU trousers (they use ripstop brand) OR or MARPAT Woodland or Desert trousers
- OD Green BDU belt (or tan/brown/black appropriate to the uniform)
- Green, pocketless T-shirts
- Woodland BDU jacket OR Woodland M-65 Field Jacket
- OD LBV, Molle vest or suspender-type LBE; PS vest.
It goes on and on, even down to identifying the specific pistol holster and canteens.
The above list is straight from the link, which also provides picture references for each item and information on where you can source it yourself.
In short, not only was most of the SG-teams uniform strictly accurate to military issue, enterprising fans have located and identified pretty much all of it and you can buy it all yourself.