It is foam used in aquarium filters, and its color doesn't matter, its always the same material. You can get it at any Petco. I have used AquaClear which is beige and comes in small blocks, like a stiff sponge. Marineland is blue, and is the one people call blue foam:
Amazon.com: Marineland Duetto 100 Foam Filter Pad Sponge / DJ100 - 4 Pack: Kitchen & Dining
This one will work. I believe all filter foams are of the same material. Some will be sheets, some blocks. Cut a piece that fits the cart, and is just a bit wider than the inner diameter, slip it in and trim it so that it is about 1 mm above the top of the cart. There are other more advanced things you can do with it, like making a
tank/plug, but even what I describe will improve the performance of the cart dramatically.
