I use ToughBuilt GelFit and absolutely love them
GelFit™ Fanatic - Thigh Support Stabilization Knee Pads — TOUGHBUILT
Reviews
Amazon.com

I also use this
Spear & Jackson Kneeler Board - Bunnings Australia
which I consider a good deal at $25

These are the dog's testes, but I have not tried them
0714 0714E Custom Fit Kneepads — ProKnee(R) | Professional Knee Pads & Tools | Floor Covering | Construction | Industrial Roofing

These are the ultimate