I have done quite a bit of forum marketing in the past. But I did overdose on it a bit, so I haven't looked at any forums - let alone search for them - for the last few weeks.
But back then, while I was doing it, I found it quite difficult to find good new forums. (That's not the case with blogs, of course. There are just squillions of them on any subject you can think of.) I concluded that the reason for this is that forums do take a long time to get up and running, and a lot of them fall by the wayside before reaching critical mass. The big, established ones have managed to get past that point, of course, so they tend to stay at the top of the rankings.
For this reason I thought things wouldn't have changed much since I last did a search. But earlier today I Googled and Binged for new forums using terms like "seo forum" and "marketing forum". While the results hadn't altered hugely, they did contain a few good new ones that I'd never seen before.
So, the list is dynamic after all. If you are looking for new forums, you just have to give it a bit more time to let things change.