The phrase itself won't be copy protected, but, as you said, the actors voice will be. Do get information on how much you will have to pay, you need to contact the Screen Actors Guild...www.sag.com.
As for where to get it...dunno. Museum of TV and Radio might be a place to start. They are in Beverly Hills.
http://www.mtr.org/
Or Google the saying and see if people have .wav files of it online.
Shane
"There's no need to fear, UNDERDOG is here!"