Nowadays, it's a fairly obscure film, not in print in any home video format. Its general availability has hovered between "not" and "barely" for the last 30 years or so. Every now and then, you can catch A Thousand Clowns on Turner Classic Movies, although if you blink you might miss it.
Listening to the interview with Gordon reminded me not only of how much I love this movie, but of how I first heard of this film: My longtime obsession with Jean Shepherd, who himself was obsessed with A Thousand Clowns, though in a not-quite-healthy way.
Some quick background for those in need of it (those who don't, feel free to skip ahead a paragraph or two) Jean Shepherd is best know for writing and narrating A Christmas Story, but my love of him has more to with his radio show, which aired on WOR in New York from 1955 to 1977. It's hard to encapsulate exactly what he did on the radio; something in the Venn intersection of improvised monologue, storytelling, and sardonic commentary on the day's events. It was done completely off the top of his head, with no notes, outlines, or anything. It is better experienced than described, so I'd encourage the curious to check out some of my Shep-related posts, or The Brass Figlagee, a podcast that makes available hundreds of his old shows.
When he came to New York in the mid-1950s, Shepherd had an overnight show that garnered a huge following among jazz artists, writers, and other Night People (a phrase he claimed to have coined, and just may have). By his definition, a Night Person was someone who probably had a day job to get up for in the morning but preferred to stay up into the wee hours, just brooding, because they were "bugged" about some inexplicable something. His monologues were a stab at trying to get at that something.
At that time, among his many pals in the nocturnal, creative set was the future author of A Thousand Clowns, Herb Gardner. They appeared together in neo-vadevillian revue, Look, Charlie: A Short History of the Pratfall (which also featured another erstwhile Shepherd BFF and fellow Chicagoan, Shel Silverstein). The exact content of the show has been lost to the mists of time, but peep this page from its program, in which both Shepherd and Gardner are listed with their respective credits. (Also, note the illustrations by Silverstein.)
Shepherd used to promote Gardner's "Nebbishes" cartoons on his WOR show, embellishing the spots (as he often did to those who dared advertise on the program) with his trademark rambling. Shepherd did not have many guests on his show--he preferred to work solo--but Gardner was one of the few, and he came on the program to promote Nebbishes in person. Gardner in turn wrote the liner notes to Shep's second LP, Will Failure Spoil Jean Shepherd?
Shortly thereafter, the two men had a falling out, and the reason was almost certainly A Thousand Clowns.