OP: "How do I get the address for a past order?"

You don't. You have to be smarter that Amazon, who removes the address after the sale.

It might be Amazon customer, but I am responsible for the sale to the individual who purchased the product. Simply Control P the invoice page and save as a PDF file. Create a folder and organize then by Date Sold. In fact, just create a buyer folder, when saved, PDF already assigns the date saved.