Kingii and africanus recently were available last fall... Tom at Marine Aquatics got them along with some gem tangs and I spoke to a loacl distributor that saw them pop up... I think the retail on the kingii's were around $6000, while the africanus were a little above a grand...
Hotamatua has also shown up here and there from Easter Island, with even Live Aquaria offering a few on their site at a pricey $1500... seems like years ago they were more available at less than $1000...
One I've never personally seen or seen available is C. narcosis...
Also not mentioned are Genicanthus personatus, Centopyge nahackyi and Centropyge debelius (although this does become available)...
Clipperton's I've never seen available, although they have one at the Waikiki Aquarium I took a few shots of and spoke with Bruce Carlson about...
