Rare is a very relative term, where I lived (in the area within 100 miles of Chicago) I doubt that anything would be considered really rare within the America cichlid realm except maybe Nandopsis ramsdeni, and that's only because of the international politics, or maybe Tomocichla asfraci, and I had asfraci twice in the last 10 years.
I belonged to the Greater Chicago Cichlid Assn, and just about anything was within reach in that club.
The only other rare cichlids I can think of are Malagasy because of that countries restrictions of exporting native species, and difficulty in successful breeding, or Iranocichlaa hormuzensus (another because politics). From Africa the genus Sargotochromis or Serranochromis mostly because they are considered too large for typical aquariums to be uneconomical to be exported (to few interested buyer).