In the Seinfeld episode The Old Man (Season 4, episode 18, 1993) Kramer spouts some nonsense conspiracy theory regarding Senior Citizen's Volunteer, when asked where he gets his information he replies: "The alternative media, Jerry. That's where you hear the truth."
I consider myself fairly well acquainted with the alternative media landscape of today be it the good, the bad or the ugly (let's be honest it's mostly the latter two), but what did the alternative media landscape look like in the early '90s, what might Kramer have been reading or listening to and what would he have learnt?
I am well aware that the internet existed back then, but it didn't look much like it does today, wasn't as widely used and Kramer just does not strike me as a tech-savvy early adopter of the internet lifestyle.
"Alternative media" has a long history in New York, going far back before the internet; the city was the center for any number of independent newspapers, magazines, and underground publications, which ranged from non-English newspapers, Communist and counter-culture journals, and pornographic news to the immensely popular Vilalge Voice, founded in 1955 and running to 2017. It was probably publications along these lines, which were often small enough and idiosyncratic enough to publish views startling at odds with that of major metropolitan newspapers like the New York Times, which Kramer was probably referring to.
New York City also featured public access television, where private citizens were granted airtime for their own programming starting in the 1970s, and in the 1990s Kramer would likely have had access to such local alternative programming through his cable television subscription. As with the newspapers, content was very idiosyncratic, and can include everything from a local band playing live to independent news programs. Most of these were very short-lived, but longer-lasting were some of the independent cable channels that broadcast locally in New York. Cable television was not a monolithic set-up, with different networks operating in Manhattan that are quite different from what we think of as "cable" today, with channels such as the Gay Cable Network (1982-2001) providing a very different perspective from mainstream networks like CNN.
Now, it's not impossible that Kramer was an "early adopter" of internet BBSs (bulletin board systems), where like-minded folks could gather to share information, play games, etc.; he might also have had a ham radio license or even tuned into "pirate" radio stations broadcasting unlicensed on commercial frequencies. So for those living in a very metropolitan space like New York, there were many options for accessing what might be "alternative media."