Try asking at hardware stores. If you have a phone camera, take 2 pictures: one of the handle where the key lock is located and one on the inside showing the locking mechanism. Or find a garage door contractor, they may sell the lock.
Main garage door or the service door. A photo would help.
Get in touch with a local LOCK SMITH" company who has garage door locks in stock.
Key lock? deadbolt lock? Please post a picture.