L.M.
A cheap pair of styrofoam knee pads can be purchased for about $5 at the big box retailers. Also look into gardening and soccer kneepads. I prefer materials that won't suck up water while you clean and that are lightweight (so they don't slip down when you have to stand up at times through the work).
A nice guide to purchasing knee pads for house cleaning: http://www.acleandreammi.com/apps/blog/show/35811160-knee...