Used ski pants, go to goodwill.
I have a pair of Timberlines for hunting. They are pretty warm for summer wear so not all rounders, but meet your request for warmth and the reinforcement on the seat and knees is Gore-Tex lined which is a pretty slick feature. I'm a Railriders fan as well, and those Bushwacker Weatherpants seem like a good way to go, they are now on my list. If you're really going to be on your knees a lot, knee pads (either tactical or construction depending on your aesthetics) seem like a good way to go. I can't imagine wearing true briar pants made of canvas unless walking through heavy cover (upland game hunting). Most I've had are very stiff and not particularly comfortable when seated with knees bent.Sitka makes some of the best hunting gear on the planet, and some are available in solid colors vs camo. Their Timberline pants have reinforced knees, seat, and even removable foam knee pads:
I'm a big fan of their Dakota and Ascent pants as well, but they don't have the knee pads.