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MY local rubber company(most any decent sized city will have one) sells rubber cord stock(for making gaskets) by the foot. It's available in an assortment of diameters and VERY cheap. Several feet might cost a couple bucks.

How much does rotor bounce depend on the material used for the bumpers?

I usually get it in 10 to 25ft lengths off ebay and prefer the red eraser type.

https://www.ebay.com/itm/E-JAMES-Rubber ... Sw241Ydo1C" target="_blank

bloke wrote: serious question in response to yours:
Don't other factors include rotor body weight, spring tension, and how-free-wheeling and freely-moving the rotors and linkage are?

How-free-wheeling should be a combination of rotor mass, linkage mass, and friction for the rotor and linkage connections (how freely-moving). But the only thing that should cause bounce is a bumper that’s so elastic that it returns to the rotor some of the energy that it absorbed when it stopped the rotor. Ideally, the bumper should deform quickly when the rotor hits it, and then return to its original shape more slowly—it only needs to be that shape in time for the next impact.

Adding friction to the rotor or linkages can damp the vibration, too, but then it’s harder (and slower) to work the valve.

I suppose a little bounce doesn’t matter much in practice.