Check your messages.. you should have the number of the bill in the messages received from the shop where you bought it and with that number go to the shop and ask for a duplicate I had the same situation after 1 week from the purchase.
If you purchased it officially, online or offline, your retailer would be more than happy to provide you with a copy. Also, in most cases with my interactions with Samsung service centers, they don't ask for the bill, as long as the device is registered with a Samsung account.
