How is the best way to run a 230 receptacle for a new a/c?

My central air unit went out so I bought a big window unit to get me through the summer. The a/c I bought is rated for 230/208 volts, 20 amps. The closest power source is the disconnect switch for the central unit which has two 60 amp fuses. Is there a way to branch off of the disconnect box to run a receptacle for the window unit? If there is then I`d only have to run maybe 10 ft. of wire. the disconnect box is a Cutler Hammer 4199H1. The air conditioner is a GE 25,000 BTU. My house was built in the late 60`s and for some reason the breaker box is on the backside of the house and the central unit is on the front side. The only other information I can think of is my central unit is heat and air, and the disconnect box looked like it was connected with 2 hot wires and 1 ground. As you can tell, I know very little about electricity. I have run receptacles before, but that was from the breaker box to where ever they were needed. Any advice, suggestions or ideas will be greatly appreciated.