My cousins borrowed our boat this past summer (5'8" draft) and had no problems with finding moorings. Generally, all they did was simply approach a mooring, tie up and wait for a yacht club/harbormaster/etc person to wander by. Most of the time, they paid nothing. Near Acadia, some of the moorings were $15-30 per night. BTW, the Claremont hotel in Southwest harbor has $30/night moorings and they are walking distance from some amazing restaurants in southwest harbor like Sips and Red Sky and Fiddler's Green.
I can vouch for cape porpoise as a good spot to stay. On their way back, I traded places with one of my cousins and sailed the boat back to Salem, MA over two days. The switch occurred at cape porpoise and it was a very nice protected and deep mooring (amazing restaurants on the pier as well). Also, in cape porpoise, you are away from Kennebunkport with all its craziness.
Gloucester and Rockport have moorings available as well. If you are a fan of wooden boats, try contacting the essex shipbuilding museum. The main shipwright there has some nice secret ideas for mooring in Gloucester if you befriend him; and that's all i will say about that.
A book that helped my cousins a LOT was this one: Amazon.com: A Visual Cruising Guide to the Maine Coast (9780071453288): James Bildner: Books: Reviews, Prices & more