How can I retrieve a lost receipt for an iPhone?

If you bought directly from Apple with the last 18 months, you can usually access an invoice through your Apple account. Start here:

https://account.apple.com/

Go to your devices and look for the invoice in the listing for the phone.

Call them and see if they can email you a copy. There are 4 Apple stores in Las Vegas, do you remember with one?

Town Square - Apple Store - Apple

The Forum Shops - Apple Store - Apple

Summerlin - Apple Store - Apple

Fashion Show - Apple Store - Apple

You're welcome