SEO is field where every one looking something unique.This uniqueness is useful to getting ranks, traffics and getting good backlinks for your websites. This uniqueness is depends upon the searching techniques which you prefer. For searching forum in the search engines like Google,Yahoo,Bing etc. There are some techniques listed below are very useful for you.

Techniques for searching forums:

• intitle:'keyword' inurl:viewtopic
• 'keyword' forums + powered by vbulletin
• 'keyword' forums + powered by SMF
• 'keyword' forums + powered by phpbb

The Another ways for searching forums in Google are: 
First Enter your keyword and Paste one of the following with your keyword

• "powered by vBulletin”
• “powered by BlogEngine”
• “powered by yabb”
• “powered By MyBB”
• “powered by ip.board”
• “powered by phpbb”
• “powered by phpbb3"
• “powered by TikiWiki"
• “powered by Drupal"
• “powered by punbb"

There are some other search engines also in which you get the forums site for your particular keywords
 
for example:

http://boardreader.com/

http://www.forumssearch.com/

http://omgili.com/

http://www.zhift.com/

http://metaldetectingforumsearch.com/

http://www.seoforum.info/

http://www.bbqsearch.com/bbqboard/

http://search.big-boards.com/

And there are some forum finder tools available in the internet markets.
I have provide some forum posting sites list. Just go for that.