Off the top of my head (don't expect any revelations here, but mostly for people wondering how to approach this type of thing for the first time):

* Parents/students hang out at schools and are probably a good referral/recommendation crowd

* Home buyers are looking for mortgage comparisons on Google (but that's probably a terrible strategy, since this is a highly lucrative segment to market to, so you should expect high customer acquisition costs)

* DIY heat pump installers will probably look at ads on /r/DIYHeatPumps