How do I get a receipt for an iPhone purchased online?

If you bought the phone directly from Apple, you have a email acknowledgement from them with links to an invoice.

If you have several email regarding the purchase, start with one that has a order number:

then look in the upper right for a tiny link to the invoice.

If you have multiple email addresses, the mail will go to the one associatd with your Apple ID.

If you did not receive a email or deleted it, you should be able to access an invoice through your purchase history:

View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support