Josh,

Here is a radio with a foam seal in poor condition. Side View of the same radio. They usually look like this (the knobs are wrong on this one, too).

I have never seen a reproduction seal but hope that some else has. 64fleetside's suggestion is a good one. I do not think that it matter is the seal is a little too big; you just wat a seal between the speaker plate and the inside of the grill.

Tim