You gotta buy them. I snuck under the wire and got mine for $15.00 but the prices have gone up since I believe. Somewhere in computer land there is info about it. Oh, here it is:
https://www.nps.gov/planyourvisit/senior-pass-changes.htm
So apparently I got mine for $10.00 but now it is $80.00 for 62 and over.
Yeah, I think we got ours for $10.00 some years ago - my husband qualified but I didn’t ;).