You can use a platform like Respondent to recruit extremely specific demographics. It's not cheap, but if you're strategic with your interview questions you can get really concrete directional signals with as few as five participants.
I'm in sales. This is going to sound shallow and tautological, but you find the people to interview for Product Market Fit by looking for the people you THINK are the ideal customers.
If you can't find your target market, you might want to consider a different demographic that you understand better. Most successful startup founders started a business specifically to solve the problems they dealt with at their last job. They understand their product market fit because they ARE their target market.
Tell me who you think your customers might be? Or ask ChatGPT what's a good watering hole for them, it will definitely come up with some reasonable guesses.
Was largely asking for all the people looking for specifics, since people were asking, and vague advice isn't very helpful when first starting out with this stuff. Like broadly, I've had luck with Linkedin messages for b2b and SEO for consumer, but mostly after the product is in an ok place. The initial users can be tough to find.
But sure, I'm working on things for parents/students, home buyers, and DIY heat pump installers.