I hate to admit it, but I used a mostly dead battery that was laying on my desk. I connected a voltmeter and set it to whatever the actual output was. Otherwise, you could put two potentiometers (variable resistors) in series with a battery, having both pots set to the highest resitance and a voltmeter across one pot. Adjust the pot to somewhere in the calibration range, as read on the voltmeter, and set the DSO accordingly. Your resistances are going to be low, so expect to have to adjust the other pot down as well, but you could burn a pot if you turn both to zero. Just a hillbilly-field fix.