Have you tried contacting Ernie?
If you cannot reach him, any bumper that has a smaller diameter than the opening on the bottom of your cue should work until you find an exact replacement.
It is very important that the bump be a smaller diameter, early cues with plastic butt plates used to break when a player slammed the butt of his cue on the floor.
I have not yet seen a Delrin butt plate break. This may be the combination of the Delrin being stronger and the cue maker choosing a bumper that does not expand enough to break the butt plate.