You could always try your local market , I would go to Derby Eagle Centre to get mine (I live near to Burton-on-Trent) As Greenman says, electric knife works well. Good hunting.
You could also try your Yellow Pages for foam manufacturers that are local to you. I did this when I wanted to replace the foam in my camera case, I think I got an off cut for about ?1. I just used a very sharp blade to cut it.