Sponge filters are readily available at your LFS or online and they come in all shapes and sizes. You already have all the other components needed if you’re running an airstone (air pump, air line) and you’ll still get the bubble effect but you’ll be adding some biological filtration in the process.
Even though my tanks are sufficiently filtered with canisters, I often keep small, DIY bottle filters running just to provide a little extra filtration and to provide some redundancy in the event a canister goes down. Though I power these bottle filters with small water pumps, air bubbles from air stones and air driven sponge filters provide another important function – they provide surface agitation, which promotes oxygen exchange. I get the same effect with the outflow of my small bottle filters.
If added filtration peaks your interest but you don’t won’t a sponge, you can take advantage of your already running airstone and create filtration by simply using a small (~16 oz), clean plastic bottle filled with some bio media of your choice and hooking the airline to it. They guys at the King of DYI have produced a nice how-to video for constructing one of these small, air-driven bottle filters.