As I said, go to any National park or monument visitors center and you can buy it right there.? I believe all you need is your drivers license for proof of age.? You can't get it over the internet, I was mistaken on that.
A 5 second Google search on "golden age pass" turned up http://www.nps.gov/fees_passes.htm as the first result with information on all the various entrance passes.? If you don't have time to do that, here's the pertinent paragraphs:
Golden Age Passport
The Golden Age Passport is a lifetime admission and discount pass for citizens or permanent residents of the United States who are age 62 or older. The pass is valid at National Park Service, Bureau of Land Management, Corps of Engineers, Fish and Wildlife Service, Forest Service, and Tennessee Valley Authority sites with admission fees. The Golden Age Passport admits the pass owner and any accompanying passengers in a private vehicle* if a park has a per vehicle entrance fee. Where a per person entrance fee is charged, the Golden Age Passport admits the pass owner, spouse, and children.
The Golden Age Passport also provides a 50% discount on federal use fees charged for facilities and services such as camping, swimming, parking, boat launching, and specialized interpretive services. In some cases where use fees are charged, only the pass owner will be given the 50% price reduction. The Passport is nontransferable and does NOT cover or reduce special recreation permit fees or fees charged by concessioners.
A Golden Age Passport must be obtained in person at a federal area (National Park, Wildlife Refuge, Forest, etc.) where an entrance fee is charged. The cost of the cost of the Golden Age Passport is $10.00. It is available only to citizens or permanent residents of the United States who are 62 years of age or older. You must show proof of age and residency, such as a state driver's license, birth certificate, or similar document.