You can use a little of the kind of stuff they fill pillows or stuffed animals with. Polyfill. You can get a huge bag of it at Walmert for about $3.
You can use the foam they use in aquarium filters, (Fluval)
You can get some pyramid teabags. Look for vids on Youtube on how to do this. "Pyramid tea bag
mod"
Unless you know someone who sews, or you have a new pillow or you know an aquarium keeper, it's going to cost you at least $3. Polyfill is super super cheap. Might even be less than $3.
You might be able to use the filler from a new/old carto. Dont try to fill the cartridge up all the way with it. Fluff up a bit of the mat to loosen it up and only put it in the top 1/3 of the cartridge. It won't leak. Just make it fit in there snugly , but it's important that the filler material isn't dense like it is when it's matted up in the carto. Fluff it up and loosen it up. If it doesn't wick good, Use less of it. Mybe only put it in 1/4 of the cartridge. It only takes a tiny bit to hold the juce in the cartridge.