I search for “$topic forum” or “$topic community” and there will typically be something in the first 10 results. There is a little trial and error involved, I check which one of the forums has been around for a while (10+ years works best for me) and still has regular activity (message volume + last posted messages)

Sometimes it ends up being a discord chat as this is very popular. Not ideal as I don’t love the discord walled garden but still better than Reddit. And if/when they pull a Reddit in the future, well, I’ll cross that bridge when I get to it.

I should add that in getting much better results for these queries since switching to kagi. Ddg and startpage had more spam/SEO/generated junk in the top results. It was more effort to find these communities on those but not impossible either.