You should submit AC Form 8060-56 to:

The cost of a duplicate Airman Medical Certificate, Statement of Demonstrated Ability, and Letter of Evidence is $2 each. Payment is made using Pay.Gov, a secure government financial website. The total amount due and instructions to access the correct account will be included with your duplicate(s). Please do not send payment with your request as this will delay processing.

If you have questions or need additional information, call (405) 954-4821 and select option 3.