All of the usual suspects, carry the cab eyebrow panel, including:
www.lmctruck.comwww.classicparts.comwww.oldchevytrucks.comwww.fillingstation.comwww.truckandcarshop.comBe advised, the removal and installation of this panel is rather invasive and I don't recommend doing it yourself, unless you have the experience & know-how. There are some blind spot-welds and unless you remove the roof cap or weld the seam, you will need to use panel adhesive, where the eyebrow meets the "A" pillar. Additionally, this area rusts from the inside, so the rust that has appeared on the outside, is a partial snapshot of what it looks like on the inside. I have seen this area "spot-repaired", only to bubble in other areas, that appeared to be solid previously.
I never advocate discouraging someone from undertaking a project, however, there are some things that have diminishing returns...in this case a good donor cab, may be in order, depending on the amount of rust and your skill-set. In any case, good luck with the process and keep us posted on your progress.