Aside from just googling "forum + your niche", try entering the phrase
"powered by vbulletin" + your niche. Vbulletin is the software platform many forums run on, including, I believe, the Warrior Forum.
Another place to find forums is the searchable forum directory site:
The largest Message Boards and Forums on the web!.
With the search function, you can look for forums in your niche or, in reverse, by searching there, you may get some ideas about niches and sub-niches to check out what you find there. They list info there about the numbers of members and posts (not sure how accurate it is) that can provide a few clues about how active the community may be.
Hope that helps someone!