How do you get a copy of a past itinerary from American Airlines?

It depends on different things. If you haven't yet flown the entire ticket, then the electronic ticket can still be viewed online. You can pull it up and print it yourself on aa.com. If you've flown the whole thing, and 7 days have passed, then the e-ticket (also called a VCR) is purged, and you'll have to write a letter and pay $15.00.