How to find proof of purchase from someone that is now in prison?

No. Whatever the background of the previous owner, in prison, dead, Apple consider that the contents of that iPad are private. In this case, they will not open those details without something like a court order.

In the circumstances you mention, probably the police, who may have greater interest in the contents, would have first call. Give it over to them; it is useless to you.

Without the unlock code, no.

The proof of purchase wouldn't do you any good anyway - he would have to take it and the iPad to an Apple Store, not you, so that's out. I'm afraid you're stuck.