If you are looking for new forums to post on, then of course you'll find heaps with a bit of creative Googling. But here's a little trick that uncovers them in a different way:
If you've joined some forums already, you'll notice that there are some members who post a helluva lot. You also start to see their usernames in some other forums you've joined.
Chances are that they've joined still more forums that you don't know about. So if you just Google their username (including the word "forum" if you wish), the search will reveal a whole host of online discussion boards. Chances are they'll be good, active forums, too. Experienced and avid forum users wouldn't be wasting their time on tiny inactive forums, after all.
Here's an example. There's a marketer called "work2bfree" who is a very active forum user. Here's what a search for her username reveals.