What material should i use for the handlebar ends on my motorcycle?

https://en.wikipedia.org/wiki/Sorbothane
Maybe?

Hello , what is your bike ? for simple riding , or motocross ?

you might be able to use something like https://en.wikipedia.org/wiki/Galfenol maybe?

Could I use Heavy Metal (something like K20, not the rockgenre)?

I thought the goal was to reduce vibration. How would a piece of steel reduce vibration?
If it needs to be heavy, then look at materials like tungsten or lead. If it needs to be wear resistant, then some steel alloys could be a good choice.
If it needs to resist vibration, then you are most likely looking at some type of rubber.

Maybe it would help to post an image of how this piece will be used? From the description, I thought you meant to use it as some type of grip.