After much searching, I have found a track supplier for the sliders. Restoration Specialties and Supply Inc. 148 Minnow Creek Lane Windber, PA 15963 814-467-9842 They have a part number Y526 that is available in different length. The longest shown in their catalog is 96". This is NOT long enough to do the two door wagon sliders. It is about 3" too short. BUT, they can provide a 108" length that fits nicely and is long enough. It is necessary to call them about this extra length piece. Hopefully this will help wagon builders.
Here's a couple of pics or the right side slider channel in place. These are not screwed if or sealed as yet. But, they fit!
they bend quite easily. I used a rubber mallet. Held the end on the bottom in it's place and tapped gently until the bottom curve formed. Held that curve down tight and tapped in the top curve. That took some extra patience. Tapped the top end in to make the top curve tight then trimmed the top end with snips to fit tight. Seems that a small screw is needed about an inch from each end. The tracks have to be glued/sealed in place, but I am not ready for that as yet.
They were a big help with my channel, which I matched up from the remnants of my old channel. Most of the other supply houses had no information on sizes. The channel is available from a few sources, C.R. Laurence is another good one, Metro Gl*** too. With my rigid vent window channel, a 72" piece gave me about 9" left over. I had to know what my cut pieces were in advance for the flexible channel so I had a little to experiment with.