LM7905 (preferably with a small heatsink) would be the simplest solution. Assuming these are computer power supplies, check inside them first. I've seen power supplies which did output -5v, but did not have it wired up.
Most of the arcade PS have -5V output, not most likely have -12V, if you have -12V, there should be a -5v output in the PS. If you dont need high current, 7905 is the best, their limit is 1,5A
Thanks guys. These are not computer power supplies, they are some weird arcade-type ones. They may have been for video slot machines or something of that nature.....not sure.
Wire up a 7905 using small caps (1uf electrolytic) between input and ground and between output and ground and mount it to the case using a mica insulator and nut/bolt that has the insulating washers on them.
The tab on the 7905 is tied to the input voltage so it must be isolated from the heatsink. Use the aluminum case for the power supply as the heat sink.
Also, be sure to orient the caps properly. Since it's negative voltage you connect the + leads of the caps to ground.