There are basically two types of knee pads in the world, proper plastic cap pads with straps and buckles and then there are these lightweight knee pad inserts that integrate seamlessly with your trousers.
Elbow and Knee Pads - 911supply
Sherwood CODE TMP 1 Senior Elbow Pads – SHERWOOD™
Särmä TST L4 Field Pants
Condor Knee Pad Insert Knee Pads
Gear, Protective Equipment, Knee and Elbow Pads
PROTEC KNEE/ELBOW STREET COMBO PACK BLACK
Breathable combat shirt for tough tactical other demanding outdoor use. Keeps you cool under load-bearing gear. Check availability and get yours now!
Särmä TST L4 Combat shirt
A good basic set of knee protectors. The US Army seems to switch the models every month, this is probably the current issue one.
US kneepads, McGuire-Nicholas, UCP, surplus
Mechanix FastFit Gloves
Särmä Cargo Pants