Arif, you can build dofollow backlinks right here on this forum in your posts, but you need 20 or more posts to be allowed to make backlinks. Here are some other forums:
Small Business Forums.org
Small Business Forum.net
Small Business Brief
Business Advice Forum *
Small Business Computing Forums
Small Business Forum
Small Business Ideas, Issues, Resources, News and Help Forum
Small business forums Australia
Young Entrepreneur Forums
"The best way to get other sites to create high-quality, relevant links to yours is to create unique, relevant content that can naturally gain popularity in the Internet community. Creating good content pays off: Links are usually editorial votes given by choice, and the more useful content you have, the greater the chances someone else will find that content valuable to their readers and link to it."
Also from the same page
"Forum comments with optimized links in the post or signature, for example:
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