Hi siouxsie, I also use aquarium foam, but it is by Aquaclear.
Hagen AquaClear Foam Filter Inserts at PETCO
A block of foam costs less than $2 and will last a long time, mainly because it needs less replacing and maintenance than the stock fluval filler, which BTW works terribly. Some use MarineLand Blue Foam. The aquarium filter foams are all similar in material, but vary in color. If you have a PETCO nearby, they will likely carry either foams or both.
Getting rid of the fluval will improve your PV a lot! Fluval wicks poorly, holds on to
juice too well, and gets compressed by the atty bridge quite easily. The foam holds up much better, wicks far better, and is less prone to burning from the hot bridge. You cut a long box-shape slightly wider then the cart, insert it, and cut off flush with the top of the cart. Add
juice by dripping into it. You can also use a "vent straw" with it if you like. There are other cart mods that use the foam too.