What country are you in?

In the US, standard channels are not manufactured with 90 degrees between the inside of the web and the inside face of the flange. You can use a bent plate or a built-up welded member to the dimensions you need if it works otherwise in your application.

Sometimes cranes use the american stadard beam (S shape).  These have angled flanges similar to the channels and the rollers account for this in their shape and design.

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

Thank you for you very helpful reply, I contacted Steel of West Virginia and have a sample on the way. The bearings I have coming from China for $7.5 a part, plus shipping/customs/hassles.

Glad that helped.  Who are you getting the bearings from?  The project I was doing was very low volume, so it's not worth a lot of hassle to source them, but might be good to have a low$ source in the future.  Maybe some customer will want to go in the mast business.

Thanks,

Jess
davisprecisiondesign.com

www.automotive-bearing.com, Catipiler had similar bearings for $15ea as well. They were stamped Made in China.

Thanks much for that.  They also listed a line of inline ball-joints.  I've searched for those to use on double-ended steering cylinders.  Don't have an application at the moment, but now I know where to go!

Thanks again.

Jess
davisprecisiondesign.com