frobozz, the ipac inputs go "on" when they get referenced to ground. (usually through a N.O. contact on a microswitch). Basically, the switch just jumpers the ipac ground to the input. This is referred to as an NPN type input.
So, to use the perfect 360, the optical sensors would have to be of the "sinking" or NPN-type...that is, they "output" ground (instead of +5V)when triggered. I cannot verify that the perfect 360's are NPN since I don't have one...
As for the +5 and ground to the joystick, get it from your pc power supply. If you choose to use an external power supply, then make sure the grounds are referenced correctly.