There are always mail order places such as Aquatichouse, Big Al's, Dr.s Foster & Smith, and Thatpetplace.
Your filter should have come with a Refillable Filter Media Container for each side. It snaps open and you can replace the spent media inside with new carbon and other granular fitration stuff. You can also DIY --- cut a piece of pond filter mat to replace the disposable cartridge if you don't want to mail order it.
If you want to take a break from DIY, pet shops usually sell generic filter pads that list the different filters they can be used in.