That's going to be a tough one. There were countless van conversion companies through the 70's/80's and even 90's making random pieces and tops for custom vans, conversion vans, ambulances, all kinds of service vehicles that needed higher tops. Most are now defunct. Many were offshoots of marine fiberglass companies. Your best, and probably only hope is to see if there is a tag from the wheelchair conversion company somewhere, maybe inside the door by VIN plate, that might still be in business. I think you're shooting in the dark though.
It begs the question, who cares? It's a great top, which is essentially a chunk of fiberglass. Enjoy it.