Purchase a purpose built unit at a local store, or online if no local source is available. I guess the proper acronym to be searching for is AVR, or AVS (Regulator and Stabilizer the same meaning as Controller). It's commonly built into higher end UPS's, the first link I find with a big box retailer in my area is a 90$ unit at Circuit City which is a 1000VA 600watt UPS with the voltage regulation feature built in. I'm sure a more exhaustive search would be in order to find something to suit your needs.