Kittery Trading Post is great, they have half a floor of the building dedicated to used and new guns. Prices are reasonable.
Cabelas has a far smaller selection, with generally higher prices.
I've not bought from anyone else locally. I bought one of my handguns on gunbroker.com, and used an FFL dealer from Standish. If you buy online, I'll give you his info.
I took my CCW licensing class through Cabelas, but I think the company is from Poland or one of the towns near it. It took one full Saturday to compete the class.
Scarborough fish and game has opened some of their ranges to the public at specific times and days, but I think it's mostly shotgun sports.