I would suggest obtain your material in 2 stages:

1. Blending PVC and NBR - the best would be extrusion at elevated Temperature.

When blending you are supposed to add the foaming agent - such that its decomposition temperature be lower than the blending temperature.

2. 2nd stage - hot-pressing at temperature slightly exceeding the temperature of decomposition of the foaming agent.    

And that is all - you need.

Best Regards,

Leonid