You're supposed to be able to pull receipts from the past 24 months on the website from your Delta account in the my receipts section.
I've had 1 or 2 receipts in the past that haven't loaded correctly (I imagine they have bugs just like any other system), fortunately, I still had the email.
The one time I had to request a copy I didn't find it hard, they don't need the actual credit card used, just the type of card. The response was pretty fast.
The actual details on requesting a copy are on the website here (under How do I obtain a ticket receipt for past date travel?):
https://www.delta.com/content/www/en_US/support/faqs/general/customer-care-faqs.html