I found a place that still sells XTS parts on the web, but I cant seem to find it now.
One of the problems of having a sheridan is the lack of replacement parts.
You should be able to make one out of some soft rubber, or I believe that you can make an older brass eagle(avenger, genesis) detent work. I am not quite sure about the BE detent thing though, but I thought about trying it when I had mine.
I am the most cynical person on earth.