If the fan is simple you can replace it with another. Just Google the fan model number to see how many rpm it has, also important if it is 2wire or 3wire fan.
If the fan is very exotic, you can replace the bearings in the fan (you have to disassemble the fan to see),
Bearing can be ordered on rsdelivers.com, just enter bearing formula like "3x7x3"
Inside Diameter d 3mm
Outside Race Diameter D 7mm
Race Width W 3mm
You have to measure the bearing with a caliper to know what to order.