Well I'm in FL but visit nw PA once in a while. There are some species I still need to find for sure up there..
I'd say where you saw a dor or one that has been killed is your best shot at finding another.
Everyone I talk to up there says they see the spotted adders in their garden or nearby where they may have placed those flat stones or logs. If you are finding red bellies and such, I'd think you are in the right area if you also add that to a close-by dor or killed milksnake.
Here's one from Warren county,PA

I may head up again this summer to find and photo another..
:Mark
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Surrender Dorothy!

crimsonking.piczo.com/

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• Eastern Milk Habitat - PaHerper88, Fri Feb 8 21:48:56 2008
• RE: Eastern Milk Habitat - antelope, Sat Feb 9 00:15:55 2008
• RE: Eastern Milk Habitat - PaHerper88, Sat Feb 9 10:04:56 2008
• RE: Eastern Milk Habitat - jyohe, Sat Feb 9 11:28:53 2008
• You just gave it away - Jeff Schofield, Sat Feb 9 14:48:16 2008
• JEFF I'M NOT A KID - PaHerper88, Sat Feb 9 20:06:14 2008
• No offense taken, now reread my post - Jeff Schofield, Sun Feb 10 00:56:39 2008
• By the way - Jeff Schofield, Sun Feb 10 01:05:07 2008
• Sorry, Jeff - PaHerper88, Sun Feb 10 10:38:08 2008
• You have only begun the process - Jeff Schofield, Sun Feb 10 12:22:56 2008
• ........19 - jyohe, Sun Feb 10 14:58:05 2008
• RE: Eastern Milk Habitat - antelope, Sun Feb 10 09:24:42 2008
• RE: Eastern Milk Habitat - jyohe, Sat Feb 9 11:24:56 2008
• Lebanon county, n/p - PaHerper88, Sat Feb 9 12:27:02 2008
• Lebanon county, - jyohe, Sat Feb 9 15:46:40 2008
• RE: Eastern Milk Habitat - zach_whitman, Sat Feb 9 23:21:16 2008
• Found in a basement,lyco.co.pa. - lbrat, Sun Feb 10 07:30:31 2008
• Found under tin, lyco.co.pa. - lbrat, Sun Feb 10 07:33:36 2008
• RE: Eastern Milk Habitat - crimsonking, Wed Feb 13 19:09:01 2008
• RE: Eastern Milk Habitat - tortlemon, Sat Feb 16 06:51:57 2008
• RE: Eastern Milk Habitat - PaHerper88, Sun Feb 17 21:02:14 2008
• RE: Eastern Milk Habitat - tortlemon, Mon Feb 18 05:05:59 2008