if you can find a small dc 5v power supply (eg cellphone charger or dc power supply for some other device) you can connect its +ve terminal to ground & use its -ve lead as minus 5 if its output is floating (ungrounded). if i was doing it i'd plug the dc adapter in & connect its +ve terminal to a ground via a resistor & check to see if there was a voltage across the resistor with a multimeter (no or little voltage means its suitable, 5v means its grounded).