Amex will let you request billing statements that are more than 24 months old. Go to "Statements & Activity" then "Billing Statements & Documents". Old statements should be available within 24 hours of a request.
I see you asked this question in the Citi forum as well. I don't know if there's a way to find old Citi statements, but when I got GE on Prestige in the fall of 2017, the transaction appeared as:
"USCUSTOMS TRUSTEDTRAVE 317-614-4887 IN"
The credit appeared at that statement's close as: "GLOBAL ENTRY/TSA STATEMENT CREDIT"