You can also get a Hammond or Heyboer that'll do the job but sourcing it will be a little tougher. Most amp suppliers pretty much stick to Twin-style 4 ohm secondaries, so you might have to go direct to Hammond (www.hammondmfg.com/claspg.htm) or Heyboer (www.heyboertransformers.com/tubeamps.shtml).
I got a 4/8/16 OT for a SF Dual Showman directly from Heyboer a couple of yrs ago--- easy, simple transaction and a great sounding OT..............gldtp99