Replacment remote for an older garage door
You can purchase from the big box home improvement stores or online resources a device which connects to the physical button (wired) of your opener. It is a radio receiver matched with the remote(s) included in the purchase. It replaces/augments the existing system, allowing more remotes along with the original for those who have working systems.

More advanced versions will also mate with one's portable communication device with the appropriate application.

Skylink is one such brand, there are others.