I've not bought a pre-made cartridge for my HOB's in about two years.
I use the polyfill that you can find in the craft department of most stores. I try to use the stuff in the rolls for my hobs, cause the loose stuff is a pain to try to get on right. But the loose does just fine in something like a canister.
All I do for many of them is once the cartridge is used up, I just carefully cut off the blue stuff, and save the plastic to use agian. Rubber bands work good for keeping the polyfill on.