Always carry a tape measure, and when you see one, ask if you can measure their roof?
Here's one I found. It's got links to a couple of roof manufacturers. You could ask them.
http://www.conversion-vans.net/parts/13 ... -tops.html
Alternately, if there are any folks on here that have done DIY conversions, like forum member
"terry", you might ask them if they know of any roof/top manufacturers who might be able to help.
Or try rvnet, same question for any members who've done their own conversions.
Sportsmobile might be able to offer some help, as they would have to acquire or fabricate different
shapes and sizes of lids to fit whatever vans their customers request. Maybe e-maill them and see
what they use, or who supplies them.
Hope this helps.
AS AN UPDATE:
You might want to investigate VW minivan tops. I saw one on the weekend sitting beside an
Aerostar, and it looked to me, like the footprints might be close.