When you say keypad I think of something controlling a door strike or mag lock. And there are plenty of suggestion above. But if you are looking for a mechanical lock here is a suggestion:

https://www.codelocks.co.uk/push-button-lock.htm

You can use one digit if you want. If they need a keypad on both sides this is the only company that I have found that makes one that is ADA compliant lock i.e. handles on both sides.