I think (correct me if I am wrong) what the OP was referring to was photos of the racers cutting open cell rear foams down to fit in front tires.
Rear tire foams used are usually the closed cell type (usually red or blue, depending on brand and firmness). For the front tires, most will take a full size rear tire open cell foam (usually white or beige) and cut about 1/4-1/3 off and squeeze it into the front tires. Gives a firmer front tire.
Hmmm...reading the original post again, I may be wrong as to what he was looking for. Oh well.