Ebay is by far the best place for things like frame sliders. My fav. brand are shoguns, since they are cheap and functionable, but feel free to pick whichever one you like.
Installing them is super easy. Just take out a motormound bolt and replace with a slider, torque down. Remember to do them one at a time.
Some bikes will have to have their fairings cut in order to accomodate the sliders. I doubt you will have that issue with a gs500.