If you want good quality black ebony, I suggest that you do the rounds of musical instrument makers, especially stringed instrument maker.
EDIT
See item 75, "Imported Timbers ---"
http://www.trendtimbers.com.au/complete-timber-list.php
I have dealt with these people in the past, they have always been obliging and cooperative.. Quite simply, they are timber fanatics.
You could try these people too, going back a few years I was able to buy some very large slabs of ebony from them.
http://anagote.com/timbers-sydney/