It's just a piece of rubber tubing, acting as a vibration damper around the lever stop. In order to measure its diameter, you'll need to remove it, although you can measure the width of the collapsed tube and use that to find the size tube you need. I haven't spent any time studying it, but you could use clear plastic replacement, since hardware stores (in the US, anyway) carry that sort of thing, or you could try automotive supply stores for smallish water hose, or you can use (two or three layers, perhaps) of heat shrink tubing, the stuff that bundles electrical wires in automobiles.
If you have access to industrial supply catalogs, you could probably find the exact diameter and tubing thickness among the rubber hoses. Here's just one example I GOOGLED.
http://www.lancasterrubber.co.uk/Produc ... er-Tubing/
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