There are many different ways to recover a missing proof of purchase, depending upon the method of payment. Credit card receipts are a great record, and many accounts are searchable online, depending on your card account.
If you purchased the device online, often a confirmation email will be available from the merchant.
If you provided the merchant personal information, perhaps they can pull up a sales receipt for you.
As the device is still under warranty, you can contact SanDisk for assistance as well, I’m confident they’ll help you.
Bob 
