I'm afraid it might not be that easy. First you need to know which species you have, smooth or rough. Fairly simple, one's smooth and the other's rough. Smooth would be the one native to the northeast. But then you need to know the subspecies, eastern or western, and as far as I know the only way to do that is to count scales, not easy or fun. Then assuming you have the Eastern Smooth Green snake, you have to find a suitable spot to release them. The only real way to know if it is a suitable spot would be to find that particular species there. If there is no green snakes in your back yard, there is a reason and and your snakes will fare no better.
So, you need to know the exact species and subspecies and find a wild population of green snakes to release them into. A lot of people would even disagree with releasing them at all and for good reason. Personally, I would try everything I could to take good care of them or, if it got too bad, euthanize them.
I know this is a real bummer of an answer but next time you'll do the research first. Unfortunately there are plenty of other people that won't and the pet-stores can push green snakes off on them.
Bottom line: Pet stores are evil...generally.

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Martin Whalin
My Email
Quotes from guys named Carl:
"Science stops at the frontier of logic. Nature does not, she thrives on ground as yet untrodden by theory."
-Carl Jung
"It is foolish to let singleness of purpose deprive one of the joy and delectation of the many wonderful sights and sounds incidental to the quest."
-Carl Kauffeld