oh! a circular bubbler/air stone. I know I wasn't the only confused one. I got a good 4 1/2 inch one for three or four dollars at petsmart. they're probably even cheaper elsewhere.
I got one that is about 4 inches wide and it is running on a 10 gallon air pump. It works all right but I think I need a bigger air pump in order to get the full potential out of the bubble disk.