I did the same thing to my tool box. Except my locks are on the side, so I mounted the track higher so I didn't have to cut it.I found it. It's called unistrut and should be available in the electrical dept. of Home Depot, etc.
Check the McMaster-Carr link provided by Bestatchess above. If you are looking for a local buy, then it would probably be a bit more difficult (probably have to go to a metal supplier.)Where's the best place to find strut that's different from the steel? H.D. had a 10' piece, galvanized for 9.90 a piece. Aluminum would probably be sweet.