I operate a digital marketing agency for sustainable businesses and have recently been getting great results using Ecosia advertising (as opposed to Google Ads or regular Bing/Yahoo ads).
Ecosia's user base already has sustainability top-of-mind while they're searching, and the cost-per-click is significantly cheaper than Google. Additionally, Ecosia's click-through rate is reported to be 5x higher than that of Bing itself! Typically you're not able to do much targeting in PPC ads but with Ecosia you're basically guaranteed to be getting in front of an audience that cares about the environment and eco-friendly products.
This is the article I used when I set up my first Ecosia ads: https://ecosia.zendesk.com/hc/en-us/articles/201531172-What-are-the-advantages-of-advertising-on-Eco...
I hope this helps! Feel free to reply with any questions, I'd be happy to help.