You can find instructions on how to retrieve a receipt from your Apple Account by accessing: https://support.apple.com/en-us/HT204088

If you use a Google Account, learn how to get your receipts by accessing: https://support.google.com/store/answer/6160235?hl=en.

These receipts are sent via email at the moment you make a purchase. This means you can also find your receipts by accessing the email address you use as an Apple/Google Account.