There is a site called Marktplaats.nl here in the Netherlands, its like craigslist but better, you need to speak dutch to take full advantage of it but you can find almost anything on there so try a translation service and give it a look. About a year ago I started looking for old valve amps to convert to guitar use. You can find a bunch of old PA amps fom 1940-60's that folks just dont want. Mainly coz they are distro line type things without low impedance outputs.
My best buy yet is a 12 watt (2xEL84) from the 1960's, it came out of Harlem railway station and therfore had a good service history and a proper 3 pin power supply, its a PTP wired balanced amp with a lovely peice of iron in it, sounds way better than an AC15. This one even has an 8Ohm output on it. Ever since I busted my mates Boogy Maveric (dont ask) he has been using it plugged into the speaker from his beloved (shit man I'm sorry). And it rocks, havent modified it yet either, the old ceramic phono input is high z. Cant get an overly distorted sound out of it on its own but thats what a fuzz pedal is for. You can really drive the output tubes without shaking the windows out of thier frames though.
I have another 2 from the same company from 1940's sitting waiting for me to fiddle with, like I said, I have a 30 euro limit on things like that, you never know if its going to be a lemon after all.