I need a receipt from a Watch I bought

If you purchased it through Apple, Check https://secure.store.apple.com/shop/order/list. Find your Apple Watch and tap View Invoice at the top.

If you purchased it elsewhere (i.e. Amazon, Best Buy, etc), it should be somewhere on that store’s website in order history.

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https://secure4.store.apple.com/in/shop/order/list

Or;

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