FOAM TYRES ? ?

[CrashBangWallop]

These questions may have been asked before but here goes.

What is the type of foam rubber that is used in the making of the foam tyres we use on rc cars ?

I know there are mainly 2 types of foam rubber - American foam and Jap foam, Jap has a higher natural rubber content but what is the rest of the compound made from ?

Who supplies suitable sheets of foam rubber in the UK for the purposes of making foam donuts ?

Any advice or answers gratefully received.