I'll be using my skills as a librarian for this one.

I went and found the paper that Wikipedia uses as a source on that quote, and the relevant language is "...McCarthy posed his own dualistic ultimatum to a handful of reporters: 'If you want...'" (p.521) Another paper characterizes the incident even more specifically with "As the junior senator from Wisconsin himself allegedly put it to two journalists visiting his office, "blahblah cocksucker blahblah." Shibusawa (the author of the second paper for those who didn't click the link) comments in her footnote that the source is a book by Edwin R. Bayley called Joe McCarthy and the Press, which in turn uses an interview with Charles Seib in 1976 as the source. That's where the trail ends, sadly, as I can't find that interview anywhere.

Based on the evidence available to me, I'm prepared to suggest that the interview that this anecdote comes from is unpublished.

Edit: I was still curious about this after I answered, so I did some more research. I didn't get any further, but I have some leads for anyone who might want to dig some more. If anyone knows where to find the institutional archive of the International News Service, which Charles Seib worked for at the time, that might have some documentation of the interview. The University of Maryland holds the archival collection of Charles Seib's personal papers, but the finding aid isn't available online, nor is their access policy. If you think you're the kind of person they might field an email from, or even the kind of person they might let see the collection in person, that's probably a good place to look as well.