It can be found in building supplies...stay away from fiberglass at all costs. You can also try a ziploc bag filled with spray foam from a can. Zip up the bag almost completely and use the spray tube (straw) from the liquid foam to spray into the small opening. Don't spray too much at one time, as this stuff expands rapidly. If it explodes out of the bag, you will have a hell of a sticky mess on your hands. While the foam is still soft, place the base of your skimmer on top of the bag. You will have a custom fit for your skimmer, and it WILL be quiet from now on.