Couldn't find one either, used ~1/8 closed cell foam rubber and cut to fit. It was a scrap from work, so I don't have the specs. But, it was on the softer side, but little cells so fairly dense. It had a low tack adhesive on one side, which made it a lot easier to keep in place while I trimmed and fit it. Cut it approximately to the correct width (a little over), stretched it a little on the outside of the curve, and bunched it up slightly on the inside. Trimmed the edges and bolt holes with a razor knife. I did both the cab to rear window section and window section to top section seals. I also put a layer on the front, between the thick rubber gasket riveted to the top and the windshield frame.
So far (3 months) seals way better then the old dried up rubber seals. No leaks from the top joints while driving through some pretty heavy rain. Now if I just had time to put in the new windshield and door weatherstrips / gaskets, I might actually be able to stay dry !