What's the best way to find ideas for niche paying side projects?

Looking back, it was really easy to build something like a domain name recommender 15 years ago or online calculators and such 10 years ago and mailing list tools 5 years ago. I had the skills for doing all of these at the time. Now is the next best time. Although I work in tech, I struggle to find small niche ideas that require very little marketing (SEO driven) and take 3 weeks to build, ship and launch. It doesn't seem so hard. Am I looking for Unicorns?