There are many kinds. Some members recommended the ATI/Luxar line. They aren't totally ugly. Read about them here Aquarium Sponge Filter; ATI Hydrosponge Bio Filters and Replacement Sponges.
(including sponge filter theory) then buy from Amazon if you can't find it at your LFS. I haven't seen them at Petcosmart.
I made one out of a pill bottle and a foam block I had. It worked, but it wasn't worth my time and effort. They're only in the $10 range with shipping. There are other cheaper ones on Amazon that might be worth investigating.
You'll also need:
- Air pump: ($7)Tetra Whisper is good. I bought a Marina 75 (heard it was quieter)
- Airline tubing: 3+ feet. The farther the pump is from the tank, the less noise you'll hear. Silicon is worth the extra pennies...more flexible
- Check valve: ($2) if, as is likely, you locate the pump below the water level. Prevents back-siphoning if your power goes out.
It adds up to about as much as a cheap HOB or in-tank, but it's great for your bacteria and is easier on the fish.