eBay is probably going to be the best bet if you want a new/different one. They were made by Bargman. There is one on there right now... just the first one I found, probably more. http://www.ebay.com/itm/AIRSTREAM-SH...6fc3d0&vxp=mtr
Vintage Trailer Supply doesn't make the entire thing, but does carry a few parts.
1. Yours looks like it may have been chromed? Can't tell for sure by the photo, but it is rust on there? Ours is aluminum and it shined up when we took it off and polished it when we were polishing our trailer. It can be re-chomed, it just will not be perfect due to the corrosion. See what your local plating guy says. If yours is aluminium, just polish it with the different stages of polish - gray bar for the first heavy cut to get the major oxidation off, then the other compounds for shining.
2. The end cap on yours is metal. Ours was a red lens because there is a light in there - VTS does carry the replacement lens, it's new, but made for this specific part. (and the light and wire if you want to light yours up.)
Trail-Lite 4 Lens
3. You can keep your original and just get the bracket which will hold your plate. That is still made. They sell it at VTS.
Bargman Trail-Lite #5 License Bracket
This photo is from the VTS website: