You want what is called open cell sponge. Open cell will not hold water like a car wash sponge, the water drains out. This allows water to flow through.
Off the top of my head, 15cm=6", or there about. You could use this for some home made sponge filters as long as it is open cell.
Check the marine or pond section for large open cell sponge, Mine comes 4"x12", and I cut it down.