Try doing a Google Search for "forum" and "your niche". For example, if your niche is bicycles, try searching for "bicycle forum".
Unless you target niche based forums to promote your products, you will not get traffic that will convert. Therefore I would suggest that you should search the niche based forums first. Also another mistake people make is they try to target too many forums at the same time. As a result they fail to build reputation in most of the forums they are targeting. Due to which they don't get enough traffic.