You mean the threaded bit that ties from the 'yoke' on the rail, through a rubber biscut then through the plenum support arm and is affixed with a nut at each end?
The part marked 'rail support' break on ya at one end or the other?
If a actual factual cannot be located via the suggested sources, you can get away with a threaded rod of proper length and the rubber cushion. Shove the rod through the rail hole, through the rubber, through the plenum support and put a nut on each end.
In it's most basic function, all that bush does is stop the rail from flopping about and dampens some of the vibration that may otherwise end up being transmitted to the rail and injectors. It's secondary function is to prevent gross movement of the whole rail which if left alone over time would tear up the rail to injectors hoses along with the feed and return lines.