Bar stock natural rubber would be hard to come by and it is difficult to cut/machine. Depending on the volume, there are quite a few rubber molding companies who could get you something to try and then mold up what you need. rubberworld.com is a good resource.
These same companies routinely...
Dynamic stiffness is the determining factor (typically 500-1000 N/mm), not durometer. Dynamic stiffness is a function of the material and the shape factor of the design. Typically a dynamic stiffness 2 to 3 times the vertical dynamic rate of the tire is sufficient to react the shock.
TMSengineer
Start with a high tensile natural rubber - ASTM M4AA624A13B13C12f17K11 This is a specification for a 60 duro material. You can go harder and reduce the strain level at the loads you are seeing to get it to live longer. However, it will be a tradeoff in ride to do so. Increasing...
TM engineer
Bonding the bushing to the inner and outer metal will help. Rubber is a fluid and will flow under high loadings unless it it constrained.
fyi
