If the bearing isn't chewed up and is just seized from lack of lube, you can re-oil it and bring it back. All you gotta do is remove the fan, peel off the manufacturers sticker from the back to get access to the rear of the bearing and drip some 3-n-1 oil or silicone lube in and spin it back and forth by hand until it frees up. You'll also want to push/pull the hub in and out slightly to help migrate the lube around. If you feel any grittiness or see the lube get black, wash it out with more lube and keep turning it until the lube stays clear. If you leave any of the burned carbon suspended in the lube, it will act like liquid sandpaper and further trash the bearing.
Once you're done, leave a bit of oil in the back of the bearing and seal it in with a piece of electrical tape. You'll have to make sure the back face where the sticker was is clean and dry or the tape won't stick. A bit of windex will do the job, just don't get any in the bearing.
[/QUOTE]And to add to this, 3n1 used to make a special 'fan oil' that's intended for fans. I used some on an external usb case's fans until they completely wore out.
Another thing you can do is open it up and find the fan at digikey, mouser or any other component distributor. Plus you can get the panaflow hydrodynamic bearing fans and upgrade it for the rest of its life.