Call or
email harbormasters of coastal towns in the areas you consider looking for a mooring. Some towns have horrendous waits for a mooring (Marblehead Harbor 20 years, while towns nearby "only" 2-4 years), others may have immediate openings or allow transfer from
current users to new ones.
In Salem Harbor, up until recently, there was no wait time as the soot spewing coal
power plant kept many perfectionist owners away. Since it was replaced by a gas fired plant the wait list for a mooring got to be 3-4 years.
Swampscott Harbor (technically part of Nahant Harbor) with its open roadstead has no wait list that I know of. Not the best place to be on a mooring but for a local boater beats a long drive to the next immediately available place.