Cabinet Drawer Slides, Glides, Hinges, & Hardware | DrawerSlides.com
has several makes of 60" ones. The fully locking Accuride ones are $563/pair. Ouch. They list one made by Ryadon that is $205, but it's non-locking. Maybe you could go with a little shorter drawer slide and just reach in a foot or so?
I think some type of floor mounted slide would be easier to install, otherwise you'll have to install plywood on the sides for the side mount slides. And the slides have to be within a tolerance of 1/16" side-side front to back which could take some fiddling depending on how straight things are in your unit.
I recently bought 9 pairs of locking RV drawer slides from drawerslides.com and am really happy with them.