Goshman make their own sponge, I think.
Could you use, say, latex foam and airbrush it? (I suggest testing the foam first, then seeing if you can colour it). You can get square makeup sponges.
This place does grey cubes of foam:
http://www.thefoamshop.co.uk/product/40 ... -FOAM-CUBEI think it will be like the foam you get in furniture, but that will have a nice uniform density. The postage costs might be a bit steep, though.
As well as the packaging foam Jordan suggested, maybe look at acoustic foam.
I suspect it will cost quite bit to get hold of foam from these places, but sometimes you have to shell out and try, if you really want to make a prop. I remember Ted Lesley's rolls of elastic that he discussed on one of his L&L DVDs. As I recall, he had to order hundreds of metres in order to get the relatively small amount he needed.