The filter foam is called reticulated foam. It's sometimes sold as fish filter foam (even though that's filtering water) but it doesn't follow that all fish filter foam is usable for air filters. It's also sometimes used to face loudspeakers.
I found a wide selection of it in the Bauhaus store in Berlin a few years ago. Unfortunately I've used up my stocks. If any reader is near there, they could do a great service to us all by putting some on ebay - what's there is mostly unsuitable.
