Narrator uses all voices which are available from Mac OS X by default. But there is more...
There is a whole bunch of English and American voices at Cepstral. The trial period is not time-limited; It never expires. However, a voice will interject reminder messages into the audio output until you purchase a valid license key for it. Each single voice has a price of 29.95 USD at Cepstral. [State: 2010-11-30]
acapela Group offers Infovox iVox which enhances the voices for Mac OS X at a high quality. (These voices can be used in Narrator and other voice supporting applications.)
These voices are distributed by AssistiveWare who have also developed the alternate product [GhostReader] which you can get here, too. There is a whole bunch of English and American voices available. The bundle price is 99 EUR for a whole voices package. [State: 2010-11-30]