hey man. my best advice (if you haven't found one yet) is craigslist. i mean, scour that thing everyday. try different categories... don't give up. hell, look at dealerships for trucks with caps on 'em. be patient, and you'll find what you need. took me over a year to find one in my price range... the color doesn't match but i'm all about function before fashion.
Just an observation, but it seems like most topper manufacturers use only a few size doors across all of their model styles. I doubt the topper is a true Ford product, it is likely a dealer installed product...but I know little of Ford from this era. So, take measurements and broaden your search?