If you work with solar stuff, how you get people interested can be different. You can either pay companies that already collect names, or make your own list by showing ads.
If you own a solar company and want people to book a time to talk, you can team up with someone who does that, or put your offer on a site that helps you get sign-ups. The price changes based on things like how many details you want and where the person lives.
If you just want to buy names to send emails or make calls, you can find sellers that offer them pretty cheap. The cost depends on how new or old the info is.
Try searching online for “solar leads that are a little older” and you’ll see a bunch of choices. I can think of a couple of groups that used to help with solar stuff—maybe check out ones like RX or PMG in the U.S. These are just ideas, though, so make sure you check everything out before buying from anyone.