you could get one from the pet store but if you have a good size tank i would get a different skimmer. LIke a seaclone or something that uses a pump to create bubbles instead.
Those plastic tubes with a wooden airstone dont really do much except get rid of the big sludge. The seaclones and others are about 30 times better.
But like i said if you want a new piece sure you can buy it, just make sure its the right type of airstone.