I have found the 'built in' pads that you use with Skillers work trousers to be quite adequate for all but the most demanding applications.
Advantages? They're always there when you need them, and you can walk normally.
www.skillers.com or
http://www.skillers.com/Skillers/For severely demanding applications - as in, several hours on ice-cold stone - the absolute best are hard rubber with foam linings. The hard plastic ones slide around too much, while you have already discovered the problems with the cheap sponge rubber ones. I'd check 'tactical supply' places like Brigade Quartermaster for them.
Try:
http://www.brigadeqm.com/cgi-bin/ta...mp;celSourceId=ALT3&celTargetId=ALT4
