FedEx is considering Boeing's 767, which lists at $167.7 million, and Airbus's A330, which lists at $203.6 million, according to the source.
Memphis, Tennessee-based FedEx runs the world's largest cargo airline and No. 2 package delivery company. Its potential freighter purchases were first reported on Wednesday by Bloomberg.
FedEx has been updating its fleet to add more fuel-efficient aircraft.
As a matter of policy, we don't discuss market rumors or speculation, FedEx spokesman Jim McCluskey said on Thursday.
Airbus declined to comment, and Boeing had no immediate comment.
According to the company's website, the FedEx Express division has 688 aircraft made by various companies, including Boeing and Airbus.
FedEx shares were off 1.5 percent at $75.01 in midafternoon trading.
(Reporting by Lynn Adler and Tim Hepher; editing by John Wallace)