Fan motors are very cheap providing you buy 10,000 at a time from China and agree to take 1000 per month.
If you can find a motor that fits your fan you'll be very lucky and it will take a long time searching and cost almost as much as a new fan.
If the action is stiff, you might try a small squirt of WD40 oil in the bearings before going to the shops.
If there's a heat-wave where you live fans may be in short supply.
Good luck...
Peccavi, July 2009