J-Channel, or as some like to call it, C-Channel, is one of those things that you will need to replace if you're restoring an Airstream. There is no way around it. Sure, some of your sections will be salvagable. Most will simply need to be replaced. I'm at the point in my restoration where I need to do just that. My curved piece was (thankfully) in good shape, but all of my straight sections had deteriorated to the point that there was just no saving them.
I was talking to Frank at Frank's Trailer Works ( http://www.frankstrailerworks.com/ ) about the situation, and he said to send him the dimensions of what I needed. I sent the dimensions on a Thursday, and Monday morning I had a package delivered with a Frank's Trailer Works return address.
Tonight I pulled out some of my old pieces of J-Channel. I wanted to get a few images of what the originals looked like for posterity. Not good.