Your most likely down to finding a used one, because I don't think anyone is still making them for WD21's anymore and likely have not for some time. Watch eBay, check wrecking yards regularly, scan Craigslist, etc...
There were two companies that I can think of that used to make these visors in the 80's/90's. Lund was probably the best known, As for the second I do not know what their name was, but iirc it was Trailmaster who bought them out years ago (maybe mid-late 90's) and ultimately killed off the product line entirely. Only reason I know this is because I used to work for a company that made them for the original company and after TM's acquisition of them, we were just waiting for TM to officially tell us to destroy the molds. They had stopped production of them before I started there and the molds were still in storage long after I left.