Here are two places. One for info and the other for maps:

GameFAQs: Final Fantasy XII (PS2) 80 Rare Monster List by sephirosuy

and

FF12MAPS.com - The Final Fantasy 12 Map Database

You click Clan Primer and then Rares and then search for anyone of the 80 monsters. Once you get the info for the monster you can click "show on map" next to the location and it will show the exact location on the map.

EDIT: oooo thats neat how the URL's by themselves went from plain URL's to a text thing...