What can I do if I can’t find my iPhone receipt?

Contact the place where you bought your phone and request a copy.

As mentioned, you can contact where you purchased to see if it’s possible to get another receipt (assuming this is a fairly recent purchase). If you purchased online you can look through your email to see if you received it that way. Or you can log into your account with wherever you purchased (if it was Apple you can use the Store app to see it. If it was a retailer you can log into that account and look for purchases).