The lack of PCB's is probably due to the fact everyone who does guitar amps knows better to wire them point to point.
Guitar amps need much more shock proofing than any amp, and p2p is the only way to go, with good turret scheme. Or a very sturdy PCB, but as far as I know any repair tech shuns them. They will break. But a well designed mechanically secured (solder + mechanical attachment) wiring is not only easy to repair, but will last a long time, if not forever.
I have not seen guitar amp PCB's for sale anywhere. But there are p2p kits. Some very high quality unique projects, like this one: http://uraltone.com/kauppa/product_info.php?info=p1413_UralTone-Jr-tube-amp-kit--version-2-5-.html&XTCsid=7c4409nr5tc7ripvgmsk7d1po3
Allows a ridiculous amount of tube choices and easy mods.
I would suggest that if you were going to build a PCB tube guitar amp, you'll probably find that by the time you buy all the expesive transformers, tubes, speaker and cabinet, you've probably spent way more than the cost of an Ashton or Bugera cheap ass amp, not that I am promoting such an amp. And what you will end up with is something that may struggle to last 10 years with the heat that it will be subjected to and the problems you may have in servicing it, breaking pcb tracks/pads as you try to replace pots etc.
If you can, go down the line of Point-To-Point amps, which are so much easier to service, you can modify them to your hearts content and it will last a lifetime. Go PTP and there are lots of kits out there.