I haven't seen THE KRAMER POSTER, but I just bought THE KRAMER T-Shirt at a men's (or maybe unisex) chain clothing store called Zachary's. The one I visited is in the Livingston Mall in Northern NJ. I think Zachary's is primarily a North Eastern chain.
But like the Simpsons shirts, I suppose it will only be a matter of time before THE KRAMER saturates the marketplace.
They also had a collage "Seinfeld" shirt with all the characters and the show's logo.
--Tara
(P.S. My T-shirt cost $15.95.)