I think what you need is called "mast rail channel" and, like you said, it is made for forklift mast rails. Most mast mfgs seem to have their own captive tooling and aren't in the business of selling channel. We finally found a steel mill that was tooled up for it, stocked it, and was reasonably priced:
STEEL OF WEST VIRGINIA, INC.
17th Street and 2nd Avenue
Huntington, WV 25703
(304) 696-8200 or (800) 624-3492
FAX (304) 529-1479
Mast rail, steel extrusions, grousers, channels
Ivan Stewart
Also spent scads of time searching for info on mast guide bearings for the same project, and none of the bearing companies were very helpful--most of them have abandoned that business or are about to, and availability was poor and prices were high. We finally figured out more or less what we needed, and found that the aftermarket forklift parts suppliers have tons of them, cheap. They've even driven the OEM people down to a reasonable price, and we actually sourced the bearings from the local Caterpillar Forklift dealer for $39 each. That was about a third of what the bearing distributors quoted. That's about the oddest purchasing thing I've ever run into.
Good luck