Check out
The Tube Store, I got a great deal on some JAN 12AX7 there.
While I'm not as tube savvy as HamishR and Vigilante398 (I've bought a drop compared to the buckets of tubes they've bought), The Tube Store has been the best I've found for sourcing tubes.
Yeeps! I just checked the JAN GE prices and they've gone wayyyyheyyyy up. So maybe not mil-spec JANs then...
Still, I got great customer service from The Tube Store, and very good CS from Amplified Parts, too.
For some circuits, and/or for bass, you may want to check out getting some 12A
U7 tubes, a little less gain than the Xes which may have Xcessive harsher distortion when you're looking for just some smooth grit.
Haven't read this, it was the first hit to find some info for you about 12A?7 series tubes:
Swapping tubes is a cheap and easy modification that can have great results. But not all tubes are equal! We put three common types to the test.
producerhive.com