I've stumbled across one or 2 hobby shops in Maine, but in each case, it was purely by luck, as they were pretty old-school, and not so big with the whole internet thing. They were both smallish in size, but here was one of the better ones in Portland. As with all things in Maine, nothing is too far from anything else. Portland-Bangor is about an easy 2-hr drive. Mostly twinned highway all the way through: http://www.mainehobbies.com/
I'd suggest expanding your search to include model train shops, as many of them carry models too:
http://www.railmodeling.com/dealers/namerica/MAINE/
Hobbytown USA has stores in Nasua and Manchester NH, which, if you're touring the Northeastern US, may not be too far off the path.
Here's another tip: Get her to check out any Marden's store she comes across. They do creditor liquidation sales of just about everything they can gets their hands on from stores of all types across the US, and I've gotten some insane discounts on model kits from them. Stock-wise, it's a total crapshoot, and varies from store to store, but when you hit paydirt, you hit it hard: Last visit I scored several Tamiya 1/350 scale Battleships for under $20 USD apiece. http://mardens.com/