You could cut it by yourself - get a sheet of foam rubber in your stationery shop and cut it in thin strips with scissors.

Remove the old foam with a toothpick and a solvent, then glue in the new one with. Let the glue dry for a moment, then apply the foam.

Works fine, it´s cheap - black, 0,5mm thick, A4 size, got it for 0,40Euro.