Get butterfly-style (not sleeve-style) if you want to wear them over pants.

Here's a cheap option, I had multiple pairs of these.

I now wear these - they're pricier but considerably better-made (they have a D30-like material in them for increased impact absorption, the hard caps are replaceable, and they have a little strip of a rubberlike material at the top that helps keep them from sliding down).

I'm not clear on what is making your shin take so much abuse but I know it happens because a friend of mine has the same problem. He actually wears a soccer shinguard on his leading leg.

As far as knees, here's a reasonably-priced good option, and a pricey one. I used to use the first one, upgraded to the second one.