Apple Watch receipt

Go to reportaproblem.apple.com. Sign in with your Apple ID and password. A list of your recent purchases appears. From here, you can view a receipt, request a refund, or report a problem with an item

Or;

You would want to contact Apple Store on the phone and ask them to email copy of the receipt to you again. You will have to verify yourself by providing some transaction details and you will want to verify precise spelling of your email. Phone number can be found at www.Apple.com/retail