You can't buy them yet. If you could, better believe I would've after a laborious trip to the USPS today. But they'll be released on June 5. (I seem to show up at the Post Office the week before the coolest stamps are released. I was in the week before the Cherry Blossom Centennial stamps were released, too.)
How Johnny Cash Became a Forever Stamp tells about the photoshoot for the Cash's Best Of album, back in 1963.
It was a good year for Cash, one of his best in a very long time. Things were heating up with June Carter, the love of his life who later helped get him sober, and his career as a singer-songwriter was finally taking off... [The photographer] wasn’t thrilled with the black and white photos he took that day, and Columbia ultimately chose a color shot for the album, which seemed just as well. But fifty years later the photo has surfaced again, this time as part of the U.S. postal service’s Music Icons series.
And evidently Ray Charles is next in line.source: USPS blog Stamp of Approval