How do I find a past invoice for data storage?

Check your purchase history. If its been less than 3 months it can be found at https://reportaproblem.apple.com

You can get the invoice by clicking on the order number there.

If its been more than 3 months, you can find it in the App Store App under Purchase History in your account.

click here ➜ View your purchase history for the App Store and other Apple media services - Apple Support

You would have got it in your email.