iOS
A receipt should be sent to the e-mail address tied to your Apple ID. If you can't find the invoice in your e-mail inbox, you can find your purchase history at reportaproblem.apple.com. Click on the selected purchase and choose "View invoice". Check Apple's support page for tips on viewing older purchases.
Android
A receipt should be sent to the e-mail address tied to your Google Play account. If you can't find the invoice in your e-mail inbox, you can find your purchase history on Google Play or at pay.google.com. Check Google Play's support page for more information.