I've seen the PS-520 sell separately from the M-520 mixer on only two occasions.
You may think the PS-520 power supply is hard enough to get on it's own. It is, no doubt. The big pitfall is the umbilical power cable, 20 pins of some obsolete connector. Not a typical D-sub.
You may find the PS-520, but you'll be hard pressed to find the associated cable, that goes between the power supply and board.