Proof of ownership
How do i get purchase receipt for a device you didn’t originally purchase. And you can’t contact the original owner.
iPad Pro
Unless the original owner (from whom you purchased the iPad) has provided you with the original purchase receipt, you'll not be able to obtain a copy.
Consider that the iPad may have been purchased from one of many retailers, or which Apple itself is only one possibility. Also consider that even if possible, a retailer will likely decline to provide a duplicate receipt to anyone other than the original purchaser - as to they will likely breach privacy laws/legislation that exist in many countries.