Some garage door opening devices are supplied with 2 or 3 remote control handheld units. These units when pressed will activate the garage door openers and the door will either open or close. Sometimes the remote control units become lost or damaged and do not work. Or perhaps you just need additional units for each of the house holders that require access to the garage.
Most, if not all, of the manufacturers of garage door opening mechanisms will supply additional or replacement remote control units, but they are expensive. There are universal garage door remote replacement kits on the market that can be attached to your existing door opening mechanism at a much lower cost. They are easy to fit and usually are supplied as standard with two or three handheld units.
If you are just looking for additional transmitters, and you have in your possession one of the original hand held remote control units, you can then purchase universal remotes that can be cloned from that original. Many remote control units work by sending a code to the receiver in the garage to open or close the door. A cloned transmitter will copy this code from the original unit and store it. When the button on the new device is pressed it will send the new stored code to operate the door. Some other remote control units will send a different code every time the button is pressed. With these remotes » Read more: Garage Door Remote Control Replacement