i used to fall on my hips heaps when i was learning to slide so i out two squares from an old wetsuit and put velco on the inside of the pants and on the wetsuit material. It stopped the rash and providedpadding too.
Protec make hip protectors and pads too.
A skate shop here in canberra has some but I don't think they have any online store or anything.
Sweet beet, check out any skating store that stocks equipment for inline hockey. They supply padded lycra shorts used instead of the bulky ice hockey pants. They've been around since mid 90's and i'm pretty sure easily puchased online. see the below link
If its just Grazes from sliding along on your ass and hips, then wear jeans and take to the wear areas with duct tape (about 7 layers). That is what I do and it has saved my ass skin so many times its not funny. It also makes your skate jeans last longer.
If its about falling and jarring your body, then don't! (People of our age cant get away with it, or at least I cant, so Ive learnt to ensure that by the time I come off im in a squat position and go for the slide).