What kind of foam is used that is placed with vacuum cleaner filters?

The foam always seems to get dirty before the filter needs to be changed. I always end us gently washing & reusing them. Is there a particular foam I can buy so I can make my own?

It is called open cell foam. The kind that I have seen that is sold in bulk is a larger mesh that is used for air conditioner filters. The vacuum cleaner filters have a finer mesh to catch finer particles.