You could contact your local NAMI chapter or County Mental Health Center they can help with literature and guidance.
Also, our area has a parent advocacy group for special needs children, they have been helpful when we started our support group.
Local hospitals are a good starting point also, ones that have a psychiatric facility. I would ask for their social worker to discuss your ideas.
The local school districts have psychiatrists on staff and local colleges have student services offices that might give you ideas.
It really depends also on the population of your group, teens or adults. Suffers or family members.
We started by advertising for free in the local newspaper and through the local hospitals website.
We utilized our local library for a free space to hold our meetings.
Hope this helps a bit.