Abrasion resistant ballistic nylon can take the beatings that are dealt. The internal padding only adds to the long life span and your comfort level. Four-way stretchable backing helps keep you cool and keep your pads on lock down so they stay where they need to be.
The Proto Proto Knee Pads are newer, so they should be commonly available, both new and used. Check out the Places to Buy links below for places to purchase them new.