The billing is handled by Apple on iOS, so you would get a receipt from them. Often, it can take 3-5 days to arrive (assuming you have not turned off email notifications from them). Otherwise, you can find it as follows:

1. Open the App Store.
2. Tap your profile icon in the top right corner.
3. Tap your name at the top of the profile page.
4. Select Purchase History.
5. Find the order and tap the "Total" label to see the transaction details.
6. Tap "Resend Receipt" to have it emailed to you again.