How to find an invoice for an app purchase

A receipt would have been sent as Zuraczurx indicates, or you can view your purchase history.

Read this support article for directions about how to see your purchase history in the App Store or iTunes Store [mobile device, computer, or web] --> View your purchase history for the App Store and other Apple media services - Apple Support

You would have been sent an email from Apple to the email associated with your Apple Store account.