Need Bill of my apple watch

This is a user to user only forum, so we can't help you with your receipt. If you bought it from Apple and it was within the last 18 months, you can open the Apple Store (not App Store) app on your iPhone and find your purchases within that time frame.

If it has been longer than 18 months, you'll need to contact the store where you bought your watch and hope they can help you.

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