You want to look for "bucket heaters" as the only ready made option. These are immersion elements (with a thermostat) on a power cord. I have the Namco 1000W one that will happily heat 5 gallons of water in a cooler to 190F plus in a couple of hours on it's own. I use it to preheat sparge water while I'm setting up, (and out at the brew store buying ingredients), but it'll also assist in bringing things to the boil. The thermostat should prevent it boiling water though.