Not sure if it needs to be steel because welding, onlinemetals has 304 stainless channel but it's god awful expensive.
Aluminum might be reasonable.
http://www.onlinemetals.com/merchant...008&top_cat=60
Didn't see any cheaper steel channel. Mcmaster also has the aluminum star extrusion that we use for building a ton of machines; its cheap and strong and can easily slide things on bolts down all 4 sides.