Check out L.L. Bean's Outdoor Discovery schools (link
HERE ). I know they have shooting at their facilities in Freeport (I THINK) every tuesday. I went a few days last year.
As far as just open land, it's iffy at best. My dad shoots at our house in Bethel, but the neighbors aren't thrilled (he does follow the laws and is well within his rights). I'd say scout it out... get out there and look. When you see a plot of land that looks good, find out who owns it and ask permission. Makes some friends who have large plots of land.
The powerline suggestion is a good one too. Check it out. As long as your licensed to do what you want to do and scout it out well enough, you should have no problems.