I have converted a couple of server PSUs and neither of them had switches. I would have just put a switch on the jumper I made except the fan runs as soon as you plug it in. So what I did was open up the case and cut the black AC input wire right after the plug and put a switch I stole from a fried ATX power supply in line. Then I cut out a little square in the back of the PSU and wedged the switch in there such that it couldn't move once the case was put back together.
This would probably work fine for you.
http://www.radioshack.com/product/in...ductId=2062523
http://www.radioshack.com/product/in...ductId=2062485
Just find a switch that is rated for 125VAC and 10 amps or better. The circuit breaker on your house 125VAC is probably only 15a or 20a, so getting a switch rated higher than that would be pointless.
Good luck! Make sure it isn't plugged in when you're working on it lol.