Q: How do I find proof of an in app purchase?

It should show on your account's purchase history (from which you can take a screenshot and/or get the order number) : See your purchase history in the iTunes Store - it's not searchable so you will need to page through your purchases until you find it.

Or if it was done in the last 90 days then it should show on http://reportaproblem.apple.com (which doesn't have the order number)