All the yzA's and B's should have them and I know you could buy aftermarket ones.
They arn't all that crash hot because it's quite easy to strip the heads on the hex very soft.
I'm in the process of making a few Titanium ones before my work shuts shop and go over to the states to have everything Manufactured.
The us dollar is real good at the moment for us so it's cheaper to buy every thing allready made from there.
There is a little difference in length between the yz360 and 250 but they will suit both etc.
They would be perfect for a Hallman replica for sure and I reckon thats were the idea came from.
All you need now is a yz clutch cover, magi cover and the clutch sproket cover and a yz250a pipe.