By coincidence, I've been doing some web cruising for info on trailers, and have run into some "misplaced links" for vans. I didn't save links on them, but the following should be easy enough to find with a search.

Leisure Travel: Vans only
Sportsmobile: Popup Van Conversions only
StarcraftCorp: vans and conversions (and handicap vans)
Winnebago: Has small units, but don't advertise them much.

They might give you a lead on where to get the pop-top to put on your own wagon.

One of the dealers in my area has had a Winnebago on an expanded "compact station wagon" (Chevy?) chassis that he's been trying to get rid of for at least a couple of years. He says the problem is nobody wants the 4 cylinder engine. (for an idea of the size).

John