Once you have a chapter going, it is a good idea to reach out to other local organizations to find out where they stand and if they are potential allies.
Local government (attending local government meetings, especially when no one else does can quickly make even a small group of activists very influential)
local houses of worship (reaching out to them will be useful, even if it just means identifying which clergy will be an obstacle to your plans, Hint: Unitarian churches will almost always be sympathetic)
progressive groups (any local activists groups will always be happy for help)
college campuses (even if you aren't a student, you can form alliances with student orgs on campus)
local unions
local social organizations (Masons, Elks, Knights of Columbus, mens and womens clubs etc)
Community centers and libraries (these will be very useful for hosting meetings)
local businesses (many small business owners are also "pillars of the community," and even if they are unsympathetic its better to know that before you come to loggerheads)
local charities and mutual aid groups (these give your chapter an opportunity to volunteer and they will know the issues facing your area)
Local attractions and events (museums, parks, bars etc, also find out about any events that are held regularly in your area like carnivals, street fairs etc which will give you a chance to table or do other forms of outreach)
Once you have a chapter going, it is a good idea to reach out to other local organizations to find out where they stand and if they are potential allies.