John,
I wouldnt say it's very few coming in.
There is one Hong Kong importer who started importing directly from xmas islands along side hundreds of Flame angels he's importing 2-4pcs Black Tangs every shipment.
I agree on the size,last couple of years huge ones are coming in , just when you think you saw the largest black tang ever, a larger one comes in the next shipment.
Im not sure how smart is it to collect those full size specimens at this rate for future generations.
About smaller specimens, like 1.5"-2", saw them on few occasions and I strongly advice to check such specimens very carefully before buying them, as LargeAngels said , they are usualy paper thin, and I'd think it's cause of the rough handling they receive.
Such size Zebrasoma spp. must eat 5-6 times a day, when they are coming from xmas island, they usualy wait for the flight to Hawaii (and they DO have troubles in that line) and then from Hawaii to the states, sometimes too long for the small Black Tangs.