I went to Petco/Petsmart. They each should hold some kind of sponge cartridge that is only a sponge and is used for other filters. You should be able to cut them down and shove a few of them in there. Any sponge bought from Petco/Petsmart should be fine!
open pore sponges.. the rough ones that are usually green. You can get a pack at the dollar store and they are perfectly safe for fish tanks (use 'em all the time).
For the filter I would make a bleach/ water solution (Very diluted) and take a toothbrush/bottle scrubber and scrub it thoroughly. Then I would run the filter (without a cartridge, etc) in a bucket of water mixed with vinegar just to be extra sure.
yup.. the sponges are better in fact. The carbon is pretty much just a marketing trick to get you to buy the cartridges. The carbon does absorb some stuff but it is only active for like a week or so, so you would spend a lot of money on the carbon when if you use the sponge it does a better job because beneficial bacteria can grow on it.
Go ahead and use carbon if you want, but I'm told you have to change them out monthly (although someone keeps them in no problem). I'm too paranoid to do that, and yes, just stick sponges in there. Mechanically filters, and also does wonders for biological filtration, as you can imagine. Just no "chemical filtration" (that would be carbon), but chem. filtration is hardly necessary.