You can get this stuff from Caravan / camper parts / accessory shops , they use it to seal windows and doors etc , also Clark Rubber have it .
Cant remember exactly what it's called but it's something like "adhesive foam rubber" , You just peel the back off and it sticks , It comes in all sorts of different sizes including the size you can use to replace the the foam behind pinball standup targets , actually you can use any size and cut it suit it the same stuff exactly :)
