How I can find my recovery key for my iPad?

It isn't for your iPad as such, it is for your Apple ID.

I'll assume you are using two-factor authentication for security. A recovery key is simply a 28 digit code that you write down and store in a safe place. In that regard it is wherever you may keep such a thing. You can generate a new recovery key from Settings or System Preferences on a trusted device signed in with your Apple ID. This support article provides details about how to do this, as well as other information about two-factor recovery keys: How to generate a recovery key - Apple Support