If in fact it only draws a maximum of 250mA at 10V or so, you wont be able to use a normal PC power supply. Those supplies are switching so you must draw a minimum load from the supply. I think the load is around 5-10A or so. But for 250mA, you should be able to find a wall pack either mail order or from radio shack. A 9/10V wall pack rated at 0.5A would probably work ok (the voltage will be slightly higher at lower loads).
Then you can use a 555 oscillator configured with a 50% duty cycle to drive a pair of power transistors to switch the current between the two units.