I replaced my 11 year old foam last year, I went aquarium shop and got some plastic filter foam. It comes as flat sheet enough to do three discos about £3.50 a sheet, there's several grades/density of foam so take some of your existing stuff along to match up although you will need the light density one, the one you can see a bit of light through, some of the filter foam is high density which wouldn't let the air flow through.