Yep, I just searched "bulk filter sponge" on Amazon, got
one like this. This might even be the one I got.
Definitely, never buy another cartridge! They're a complete con. Cut the bulk bought sponge to fit the size of the cartridge plastic and hold it against the plastic bit and slot it in.
@Uberhoust is right, you don't need carbon in the tank on a day to day basis. Only really needs adding if you're struggling to get the water clear, or if you've used meds or something, and the carbon will help clean them out. He's also right that if you have some in your filter already, then it's maxed out it's capacity for absorbtion after a month or so, but will still work as filter media for the bacteria to grow on, so no need to throw it out unless you want to replace it with sponge/floss/something else.
Check out this video for ways to maximise your filter and get rid of having to buy cartridges! Really useful video
