Why are you looking to spam the forum? I suggest making lots of posts yourself at least 10-20 a day. And just wait, if you have good information they will come. Also look for mods for SEO on your forum to help the process. Good Luck
I don't think TS is thinking of spamming the forums though. Depending on your niche. You can find tones of niche forums on every niche at Big Boards. You can also rank them according to the number of members, traffic and language. Good luck.