I recommend a Champ 12 OT ( 5k - 8 ohms )( Fender part# 024038 ) , beefier then a stock Champ OT . Did get one in the Netherlands from my parts-supplier .
You can try the Hammond 125SE , similar OT 
-------------
The One Electron UBT-2 SET transformer is very nice, too. Hefty. Takes a licking and keeps on ticking. Nice brown, brown tone.
 
Site:
 
http://www.one-electron.com/trans.html