Best reference I know of is here: http://www.audiotubes.com/12au7.htm
Just a list of equivalent tube numbers without descriptions:
12AU7 / 12AU7A
I have several of these (underlined) and have tried them at least once in the amp (with varying results).
Mullards, Telefunkens, and old American black plate tubes sound best to me.
The recent remade Russian and Chinese tubes sound noticeably worse, even in this amp.
I think if I couldn't get any vintage tubes, I'd probably not bother to use this amp at all.