I wouldn't use just any sponge in the filter . There are sponges sold for AC filters, but you can buy bulk sponges from places like Big Al's Online and just cut them down.
Stick to aquarium sponges. Fish stores sell many varieties. I have used the Fluvavl replacements in the past in place of a whisper cartridge as it can be jammed in place.
Regular household sponges are often treated with anti-mildew agents and thus could leach something into your aquarium.