Lots of manufacturers on various models fit them as standard. You can purchase them via intaweb or make some, basically it’s a bar of steel or lead that slides inside the handlebars normally around 5 or 6in long and can be retained with a drop of silicone . There are lots of write ups on Google , and not so long ago on this forum addressing handlebar vibration..
I did a lot of experimenting on my CL 450 as I was experiencing hand numbness after a short distance as well as totally useless mirrors. A combination of new handlebar mounting rubbers and home made bar weights made a great improvement ,but the ultimate result was removing the top engine mount on the cylinder head. A week of running around the mountains in Spain and no indication of vibration however vibration on my feet is a different story.
Mike
CL450