HEPA filter sheets are available, but I wouldn't recommend it, if your AC is an inverter, it probably already is an air purifier, as haven't seen any that didn't advertise that feature. I did use for a very short period as tester, and didn't notice any difference or faster air purifying. More concerned about them putting a strain on the motor and restricting air flow. You can buy them in cut sheets or roll of, at HomePro, or online; Shopee & LAZ. Just keep the filters that come with the AC clean, and get regular AC cleaning done. For us, the main used AC needs it about 6 month intervals, so we get the other one done also, since they're here.