http://www.westwear.ca/index.htm
Skillers is the best work wear I've ever used. I pay about $80 or so for a pair of their work pants with knee pads, and I get about 1 1/2 years out of a pair, worn everyday. To put it in perspective, a set of Carhart pants ($70) will last me about 5-6 months max, and be in bad shape for a while before I pitch them. The best quality jeans last 3-4 weeks at most.
I would use them for airsot with no reservations, there are no BDUs out there that are as tough as these. They do use alot of Cordura in the areas that wear poorly in general, like pockets and knees. I work in commercial and industrial sites, and do alot of crawling around in areas with alot of steel and concrete. Very hard on clothing.