To disagree a bit, it seems like on the really upper end of the hobby, the fish are still reasonably popular, and the prices have dropped. Good example is the white cap goby, formerly a $1000 fish, is now for sale for only $279. Blueline angels have been quite popular recently.
But away from the big budget collector level fish, definitely agree. Cool, interesting fish are rare these days. I don't think I've seen a Butterfly I would be interested in, in several months... I'd be willing to bet, a lot of the lack of supply, stems from the front-side of the chain, rather then the stores/retailers (the tail end). I'd bet there are much less people out there today, collecting these fish, for 2¢-3¢ each, then before. Those that are left, are probably larger operations, that do focus on those "bread and butter" fish, rather then out exploring for the oddball fish.