I have had good luck with these:
Just a simple knee and shin guard. No knee stabilization - Fox Racing Titan: https://www.amazon.com/Fox-Racing-Titan-Off-Road-Motorcycle/dp/B008YAVIOK
Fox also makes these if you want knee stabilization - Fox Racing Titan Pro. https://www.amazon.com/dp/B00E8UN6CW/ref=cm_sw_r_cp_api_i_Ds0lFbFVSW3GT