If you want ones that are integral with pants, I would highly suggest Crye Precision. They have a plastic cap on them that protects from sharper things like rocks in both abrasion and just as a shield agains things hitting your kneecap. The caps have internal straps to adjust the proper height of the pads/caps. They are pricey, but there are sights where coupon codes are available.
I have a bunch of 5.11 pants and have the knee pads and unless your range is almost all grass, they don't do much. Once you have rocks, gravel or even hardpan, they aren't worth much more than wrapping a towel around your knee.
The Arc'terx caps can be found for less, but $60 is about the going rate. They don't move that much. Again, it's the plastic caps that are key. If you are on a grass range, I haven't felt the need for knee caps/pads. I will say that once you got a few bruises on your knees, you'll wish you had good quality caps, no matter how much you try to tough through it. In a multi-day class, it can help you concentrate on shooting and not feel the pain of certain shooting positions. Something I learned the hard and painful way.