How can I find the invoice for my iPhone?

If purchased online at apple.com with your Apple Account, you may be able to find it here:

Viewing & Changing Orders - Shopping Help - Education - Apple

Retailers do not keep copies of every purchase for very long, so it is always recommended to keep the purchase receipt or possibly review your emails if you provided an email account for the invoice to be sent to you.

Keep your receipt after you buy an Apple device - Apple Support

If you didn't purchase it online then the only copy may be the paper one you received when you bought it. I scan my receipts, particularly since the last one I got from Apple was a really bad print and I doubt it will be legible in two years (even now after 2 months it is hard to read).