What is the grommet called for a pipe clamp?
I would go to bicycle shop and get the rubber that goes around the handle bar for mounting lights and accessories or handle bar tape. 

Or just use rubberized electrical tape. 

Just so this is an actual answer to the question

"What is the grommet called for a pipe clamp?"  

That would be a Pipe clamp grommet or pipe clamp cushion. 

Perhaps this pipe strap dampener strip will work.

Oooohhh i got it. Cush-a-strip !

Help i cant stop.   Tico clamp strip.