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Probably a rebuild kit. http://www.aquariumguys.com/airtuneup1.html For small air pumps, it might just be better to buy a new one. Depends on how DIY you are, and how valuable your time is.
I have fixed these pumps a number of times when they stop pumping air. Get the tune-up kit and you should have it fixed in just a couple of minutes. I usually order them from Big Al's Online. Here is a link you can use.