Hi naboo,
You can use any old PC PSU and the 'trick' in using it without motherboard is in making jumper connection from pin #14 (green wire, 'PS-good') to pin #15 (black wire 'Ground', 'COM') (if old 20-pin 'ATX1' motherboard connector) or pin #16 green wire to pin #17 black wire (newer, 20+4...