It's impossible to find.
I committed to buying a 900 disc from a VP at SRAM/Zipp (he posted to slowtwitch) on the condition that he find me a track axle. I paypal'd him the money and everything. He was like, "Sure. I'll pick one up at work tomorrow..." He couldn't find one.
If he can't find one, no one can.
I also contacted Rich at Wheelbuilder.com who has some really hard to find stuff (LOOK 496 seatposts and stems), but no 900 track axle.