Hi there,
I live in Southeastern PA and I used to live in Northeastern, so I can give you some good pointers. In SE PA there is a place called Hawk Mountain- it is near the Cabela's outside of Hamburg. There are multiple rattlesnake dens there, as well as copperheads. In NEPA, if you go to the border region of Luzerne County and Wyoming County (Tunkhannock region), there are lots of copperheads and Timber rattlesnakes.
I used to find Dekay's brown snakes all over, but haven't seen them in about two years. Anywhere you can find a nice stream or pond, you are likely to find water snakes and garter snakes.
Good luck, and if you want to get in touch to do some field herping together, feel free to PM me.