Forums like this one are fast becoming great sources of traffic. This is because forums exist for virtually any topic. So whatever you are selling, rest assured your target market congregates somewhere online.
The best way to find these forums is through the search engines.
For example if you are selling,
1)cat food you would do a search for
Google=Results 1 - 10 of about 5,270,000 for Cat forums
2)Selling motorcycle parts search-motorcycle forums
Google=Results 1 - 10 of about 924,000
3)Used Cars serach for used car forums etc use your imagination.
google=Results 1 - 10 of about 4,200,000 for used car forums
Happy forum hunting