Erm...
Someone can correct me if I cam wrong.
But the AY12 was a Nissan race only head which was cast in very very limited numbers, I also think that it was only used in very few race cars. It is not hard to find one in South Africa but everywhere. You maybe able to find one in Japan but you need contacts to direct you to the correct peoples and as user S20 has said on here you need very deep pockets.
Perhaps look at the Mazda cross flow head conversion, there is a big thread on the forum.
That is my two pence worth.