I bought my knee pads from from the snowshack in Pamps earlier this year
Good ones too, they have a section cut out at the back of the knee to stop them
a) being uncomfortable when you knee is bent and
b) stops the sweat building up there. I think I paid about a tenner for them.
Best buy of the week, for the first time ever I was bruise free and they've been used several times in Aviemore here since