Re: "I need to find my paper purchase receipt for a Belkin screen protector that I bought at an Apple Store in a mall."
If you kept it, look in the packaging that the screen protector came in, to see whether the receipt is safely stored there ... (If you have your product's original packaging, you can, at least, check the barcode to find the serial number.
Also: check in physical filing system under warranties / guarrantees / receipts.
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Re: "How can I find a copy of my actual physical receipt?"
You may want to search for the receipt in emails you received from Apple around the date of purchase. Apple often offers to send a copy of your receipt by email when you make a purchase in-store.
Also: check in iOS filing system under warranties / guarrantees / receipts.
If you paid using a credit / debit card, you may find some useful details (eg dates / amounts) on your card-provider's statement.
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Check whether you registered your purchased product with Belkin online:
https://www.belkin.com/product-registration/