I assume that you are asking about the C channel that is the chassis.
The pieces that go crosswise are generally called cross members.
That particular thickness of those C channels is really thin and plus they have the big holes cut all in them. That will be pretty hard to find.
A tip that I got was.. the most generally available 5" C channels are at 6.7 lbs per foot and 9 lbs per foot and are available at steel distributors. Those you can get.
I used the 6.7 lbs per foot (C5x6.7) for a new frame, but don't know how appropriate they would be for mix and match with an existing frame.
What worked for me, and I live near Charleston, SC was..
Google, "Steel Charleston" and called some steel distributors. It is not very expensive and the one that I talked to would even cut it for me if I wanted.
As I grow older, I pay less attention to what men say. I just watch what they do.
- Andrew Carnegie