Unusual Wright WLA12a

Definitely look into the 12SX7 variant.
I used these in my Wright Preamp for years and never went back to 12SN7s.

Read here to start your research... http://www.diytube.com/phpBB2/viewtopic.php?t=4984

You might want to check out any forum or advice that comes from Maple Tree Audio. The guy that builds the amps Pre and Integrated uses 12SN7s.
Other than that I think you'll find that many of the better 12SN7s are a lot like their 6 volt counterparts. For instance I had and gifted a friend some NOS Tung Sol round plate 12SN7s which are a lot like the 6 volt tubes but easier to get and much less expensive.
I would try older Sylvania, the Tung Sols and RCA, to start. You might want to ask over on Tube Asylum of the Audio Asylums as well.