Anglers Workshop in Woodland Washington. Phone number 1 800 278 1069 and they have a web site. They call the product rubber cork RC138. You can find the same material from industrial suppliers as a rubberised cork gasket material.
Anglers has some other options available also.
You can get 1/2" thick rubberized cork rings from Anglers Workshop. Clamp it on the end of the lower grip and then turn it down. The rubberized cork is a little more dense than cork so it will take a little added effort. These rings make a nice end cap that after you have cast it for a while takes on a polish from the oil in your hand.
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