Where to buy locking seal for cabover window glass

I think typically that seal isn't what primarily is keeping the water out (or not), but rather there's a polyurethane adhesive seal between the glass and the frame itself. The rubber piece is to hold things in position, and make them look pretty, and probably to keep too much water from collecting between the glass and the frame in general.

It would probably help in finding the rubber material if you could suss out the manufacturer of the window itself (on my RV, for example, they're Hehr windows).