I like the concept of "greater Plaistow!" The best I've found in the region is up in Manchester at Bellagio Pizza. Others on this board have previous gone through all of the details -- I've found them outstanding.
Ha ha. I grew up in a big city and for 16 years I have been in S NH. I call it Downtown (tongue-in-cheek) because we live in one of the little towns near there and that is where we do all our shopping. Not in Haverhill which has a pretty good downtown. :)
Romano's pizza in Salem, NH on Main street has it on their menu. I don't know if it's "Montreal" style or not. Might be worth checking out if you're in the neighborhood. http://www.romanospizzanh.com/menu_.html
They also use cheese sauce at Red Arrow which really makes it not Poutine, at least for me. I'd be surprised if people here didn't know what it was, I thought NH was one of the few places where you can actually find the stuff outside of Canada.