A common method is to use a transformer to convert the mains into a lower voltage then rectify,smooth then voltage divide. Use a variac to vary the mains voltage and calibrate using a multimeter. If you use a AC output plug pack style transformer that frees you from worrying about the high voltage side of things. You can use the rectified volts to power your electronics but be aware this will affect your readings - do some tests to measure the effect.

Cheers

Russell