I use Hatch XTAK kneepads.
Amazon.com: Hatch XTAK Knee Pads, One Size, Black: Sports & Outdoors
You can get them in OD, Tan, Black, and some camouflage colours. Overall, they're really affordable (under $25), and because they have that X style shape instead of a O, they tend to bend more with my knees, and hug them so they don't slide down. Granted, in a 8 hour game, I probably readjust them 2-3 times because my pants ride at different lengths, but it's not an issue if you're stopping to figure out a strategy/plan.