SkyMiles Members: You can find your flight confirmation number for past trips on your Account Receipts page: Account Receipts.
Non-SkyMiles Members: Please refer to the flight confirmation email you received at the time of booking.
If You Did not Book with Delta: Contact the party that issued your booking to obtain your confirmation number.
For Assistance: Try searching your inbox for emails from wecare@delta.com or baggagecares@delta.com and respond to those for help with your confirmation number.