I have a small old tank with a built in filter that I need to replace the filter media in. I don't know the brand name of the tank - it belonged to a relative before they gave it to me - so I can't just buy a replacement like I normally would. It has a little cartridge and I was just going to take out the spongey foam piece and put another into the cartridge but I need something that will fit. Can I buy this foam stuff somewhere and cut it to fit?

Fluval makes all sizes of foam sponges. Most are available at Petsmart and are fairly inexpensive. Easy to trim to size.