Help ! About Proof of Purchase

If Apple is looking for proof of purchase to unlock an account or something similar, it will not work as proof. If you purchased from an Apple retail store, you can call them and provide the serial number of the device and a date range it was purchased in and they can provide the invoice. If it was purchased from the online store, you can call them and they should be able to find it as well. If you bought it elsewhere, not sure what you can do.

I am only 100% sure of Canadian Apple stores but you should be able to go to any retail store in the country you purchased it in and they will be able to find it. They may require ID and the computer to physically be there depending on their policies. The sales and Genius Bar employees may not have access. It may require a higher up employee than the front line (Lead Genius, operations staff, Genius Admins, managers, etc.).

You may not even need that. You can check your serial number to see if it shows coverage: Check Your Service and Support Coverage - Apple Support