I use a 404, and the sponge prefilter rack holds two long rectangular sponges on each side. Since I own three 404's, I bought some extra sponges when I ordered. I figure that in the life of three filters, I will use them up.
Looking at the schematic for the 304, it looks identicle in design if not in size. No?
Oh- got them at bigal's and petsolutions.com