I have only seen elevator bolts available from McMaster-Carr (
www.mcmaster.com) However, the smallest I saw when checking their website tonight was 1/4-20. That 3/16 you are looking for will be either 10-24 or
10-32. If I see/find anyone carrying them I'll post back.
Elevator bolts are usually used in places where you need to spread the load out over a large area and have minimal space for tall fastener heads. I am sure you can improvise with something. Since I am not familiar with your application I cannot make many suggestions.
Have you considered taking a Tee-Nut and Loctiting a #10 stud into it? That would be very similar to an elevator bolt.
Tee-Nut page at McMaster:
https://www.mcmaster.com/tee-nuts