Where can I find a rubber sleeve for the trap pedal? Also, is there a spring inside the sleeve, and if so, what’s the best way to attach it? Thanks 
You could actually use some duct hose. It is a hose with a spring molded into it. Some sort of duct hose appears to be what was actually used in the films.
I’m using some duct hose I bought from McMaster Carr in mine:
These are WIP pictures of a GB2 style trap pedal.
I epoxyed mine in place to the pedal and pedal body using slices of PVC pipe which were just slightly narrower in diameter than the internal diameter of the duct hose I bought:
Both slices are also just slightly taller than the maximum distance the pedal can “pressed” down, as a safety measure to prevent the pedal/hinge from breaking when it’s stomped on.
The duct hose is fairly stiff, and needs to be bought in 5ft lengths from McMaster-Carr. Simply search for duct hose and you can get a list of the available items. I used the 53145K32 variety in my pedals.