Senior Passes can be purchased at any federal recreation site that charges an entrance or standard amenity (day use) fee. Proof of age and residency is required. A complete list of sites where the pass is available can be found here: Pass Issuance List. Please note that USGS field offices DO NOT issue passes.
They can also be purchased online or through the mail from USGS. An additional $10 processing fee will be added to the price. For more information go to https://store.usgs.gov/recreational-passes