You know I was just noticing the baseball "slide" shorts at the store the other day.. thinking they too might work nicely.. football padding seems like it may be in some of the wrong places, but baseball players do slide on their hips
I however, picked up a pair of skating crash pad shorts on ebay a few weeks ago that fit me for less than $10... super score for me on that one! Ironic because I wasn't looking when I spotted them, but I won them with a single bid (no one else wanted them I guess).
I have a pair of thin gel knee pads (meant for volleyball) I wear skating sometimes.. they were cheap, and they're very effective - I actually bought them for housekeeping, since my knees can't take getting down on the floor without protective padding any more.. and it seems like I am constantly pulling things out from under the sofa and beds.