How can I get from Apple an invoice from a purchase made 2 years ago?

I made several purchases from Apple in the last 10 years or so. Many iPhones, plus iPads, Mac Books, etc., not just for me but for acquaintances from overseas. Now I've been asked for an invoice from an iPhone I purchased in Dec 2016 and I can't find it. When you take your device for repair they right away come up with the purchase information. It would be nice if one could get an old invoice online and print it. Maybe this option already exists but I couldn't find it. If somebody know how, please let me know. Thank you.