Just to be clear, I'm looking for a knee pad to protect against impact such as when I'm diving and sliding whilst fielding. Not a knee support as most are suggesting.
Here's the problem, any such pad may offer protection but will cause you more damage in terms of restricting your knee movements. Knee damages are long [forever] lasting so be very careful about what you use. Why not work on the technique instead?
You can also try trousers with some thick material or little bit of padding around knee area, it may help.
Think long term mate and avoid diving unless you really have to. I will be super wary of using anything tied to the knees, even the neoprene stuff.
May be someone with more experience can shed a bit more light.