As all purchases are done through the Apple App Store or Google Play Store, all receipts and purchase information will come from then.
When you buy an app, you should automatically receive a receipt to the email address on your account within a few days.
If for some reason, you never received a receipt, you can access your purchase history to get that information again.
If you are looking for a sales tax or VAT invoice, then you need to contact Google or Apple directly.