I bought one a few years ago from here:
http://www.oldradioparts.com/2a2fl.txt
I don't know if they still have them.
A much better option is to build a new ballast system with individual resistors, as detailed here...
http://www.antiqueradio.org/motvt73.htm
(thanks Phil!)
Since even the NOS tubes can be bad from age.
I have a bunch of info about the ballast tubes I posted here a while back, I'll try to find it if you're interested.