Vintage Synth Repair

Hi there,
I recently bought a Roland M240 Line Mixer, but the power supply 
(ACC120) was missing. I first thought it would be a piece of cake to 
find one on the net, but it seems not to be. I'm now looking to find an 
equivalent or to build one.
It must supply a DC symmetrical 21.5 volt (i.e. +21.5V / 0V / -21.5V) 
at 500 mA.
So, does someone have:
- a living or dead ACC120 to sell at reasonable price?
- the schematics for the ACC120 or an equivalent?
- an idea to solve my problem?
Because of the low needed current, I also thought twinking a standard 
PC power supply (most are able to yield at least +/- 12V @ 3A = 36 W, 
and I require less than 11 W). I know SMPS (SwitchMode Power Supplies) 
might behave nastily when twinked; does someone have ever done it?

Thanks,

Francois