The Hunt for the Law of Infinite Cornucopia
The obvious guess is that it comes from an obscure article of Kolakowski's that is not available online. My other guess is that it is lurking somewhere in his multi-volume classic Main Currents of Marxism, of which I have read chunks but do not have copies handy.
Despite the lack of original sourcing of Kolakowki's Law on the Internet, it appears that the person who originally brought the law to the attention of a wide audience was the historian Timothy Garton Ash, who referenced it in a 1996 article in The New York Review of Books and then again in a book of his. There is no footnote in either text.
This, then, seems to be one of those cases in which an entire superstructure of references is built on a rather flimsy base.
My curiosity growing, I e-mailed Garton Ash today inquiring where Kolakowski first articulated this elusive but profound law of political debate. Of course, it's entirely possible that the source is obviously available on the Internet and that I missed it. Any Kolakowski scholars in the PJ community? If so, please e-mail me.
I shall report back to you.