If you REALLY REALLY want C (or U) channel,
McMaster has it as usual (example, part# 1630T13). I have to suggest square tube... its very strong+light.
If you have access to a CNC mill (and a pipe bender, though it's not completely necessary) you can also try round aluminum tube (available everywhere pretty much).
I know 1501 was very successful with their steel frame lifterbot this past season... sorry our robot was too long for your amazing elevator!
-q