B2B or B2C?
For B2C, Reddit has always worked very well for me if you browse subreddits in your niche and engage in relevant conversations.
However, it's very time-consuming to manually find these so I really recommend Surfkey.io by @johancutych to always find relevant threads.
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I found some advice about this topic.
MicroConf:
(MicroConf is a community of SaaS founders)
Ycombinator:
Hope these help!
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