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.
Are you talking about the de-nitrite stuff? If so, all you really need is something for the bacteria to live on/in. The more surface area the better. If you can't find any of the actual media, I would suggest getting some lava rock, the pumice kind that people put in their gas bbq's. You shouold be able to crush it fairly easily with a hammer and then put the crushed pieces into the media bins. Be sure to rinse the lava rock before you put it in the tank and make sure that when you buy it you don't get any that is treated with anything. Just plain rock will do.