I've got 2 guitar amps that run 12AX7 preamp tubes. For the power section the Peavey uses 6L6 and the Boogie uses EL34 or 6L6. There's only one store in the Denver area I've found to test them. For music use, though, I generally buy 5-6 sets online and compare them in the amp and select on sound. It's amazing how different same-brand tubes sound.
Source for tubes? Russia. Company called Sovtek.