Both organizations are funded by and rely on donations, membership (investment) drive and fundraisers.

A Chamber of Commerce is a form of business network. It is designed to promote and protect the interests of its owners while advancing their community, region, state or nation. Business owners in towns and cities form local societies to advocate on behalf of the business community. Local businesses are members and they elect a board of directors or executive council to set policy for the chamber. Typically only members of the Chamber of Commerce received benefits. 
                 https://en.wikipedia.org/wiki/Chamber_of_commerce
                 http://www.acce.org/whatisachamber/​

The Main Street organization is the name of a community revitalization program begun by the National Trust for Historic Preservation in the 1970's. Main Street focuses on a holistic approach to revitalization based on the 4-point approach of design, promotion, economic vitality and organization. Businesses and individuals are investors, regardless of their location. The investors elect a board of directors that set policy, advocate, plan, fund-raise and oversee personnel. The board is a working, functional board that understands its roles and responsibilities and is willing to put forth the effort to make the program succeed.  Any business within the community's main street district can receive benefits. 


http://www.mainstreet.org/main-street/about-main-street/main-street-america/the-movement.html

Bloomfield does not have a Chamber of Commerce, so...

What's the difference between a Main Street organization and Chamber of Commerce?