T.B.
If you go to babiesrus.com they have pants with built in knee pads. The company Snazzy Baby sells knee pads by themselves. Here is a link to a site about them http://babyproducts.about.com/od/infantgear312months/gr/s...
You can find them at babyproofingplus.com plus some other alternatives if you just type in knee in the search box.
If you don't like those, you can also go to any search engine and type in "infant (or baby) knee pads" and a bunch of sites will come up.