I wouldn't go the route of commercial cleaning sponges for filter media. Filter media are designed to allow water to flow through them, trapping large particles to remove them from the water. Sponges are designed to absorb water instead.
Also be aware that the majority of sponges have some type of soap in them, and wouldn't be safe anywhere near your aquarium.
If the LFS doesn't have any filter media for even the most common filters, and you don't have any pet chain stores, I'd look to buy what you need online.