You can also make your own, if you happen to have some isopropyl alcohol to hand. I got mine out of a CD cleaning kit.
You need to get a floppy disk that you don't need, and take out the actual disk. Then get a sheet of paper and cut out a circle the same size as the disk (with a hole in the middle for that metal bit from the centre of the disc to go through.) You might need to use two paper discs.
Put the casing back together with the paper disc in it, pull back the metal cover thing (assuming it survived you taking the disk apart) and spray some of the alcohol on the exposed paper (both sides, to be sure.) Put the disk in the drive and it should hopefully clean the heads.
I came across this on a website (can't remember the address.) I had an ancient Powerbook and I needed to reinstall the OS but the floppy drive wasn't working, it was too late at night to get a drive cleaner so I tried this method and it worked :-)
(I should point out that this is best used as a last resort if you are certain it's the drive and not the disks, I wouldn't want you to kill a fully-working drive using this method!)