I got them on Amazon. Search 60" Full Extension Drawer Slide. Mine are 500lb capacity and sell for $255. They are really heavy duty and operate very smoothly.
I built a frame and welded the slides to the frame and then attached a 60" by 70" wide 1/2" finished ply sheet to the glides. My trailer is 7 ft wide so I actually had to rip two pieces of ply to get to the 70" width.
I took two pieces of 1X6 poplar laminated them together and mounted them on the sides of the trailer (actually bolted them through from the outside). Then I mounted the frame to the rails. The whole thing will come out in one unit by removing 6 carriage bolts. I wanted to be able to quickly remove the unit if I ever want to haul something big. The rails, of course, are permanently mounted, but they only come off of the walls two inches. I have two pins mounted on the end of each rail to secure the slides during transport.
If you want photos, I can take some when I get back in town this weekend.
Hope this helps - I really spent a lot of time thinking about it before I did this project. All my buddies say I overdid it, but most of them don't fly jets and don't know how much $$ we have riding back there, LOL.