White Rose Hobbies -- their website is www.dccinstalled.com
I have been there, small O gauge selection, lots of track and stuff like that, though. Guy is just starting out, very friendly.
As far as Gettysburg, Tommy Gilberts is a great hobby shop:
http://www.gilbertshobbies.com/
Again, small O gauge selection but awesome supply of other stuff and a good old "Like I remember" hobby shop.
Then there is Toy Trains Unlimited in Hanover, PA, just between York and G-Burg:
http://local.yahoo.com/info-12...ns-unlimited-hanover
This guy has tons and tons of O-gauge stuff, fairly well-organized, good prices, but you might want to call first, his hours seem to be very flexible. I have literally driven up and the lights were out during business hours but when he saw me try the door, he turned them on and let me in.
There is another Gettysburg shop out near the Seminary School IIRC it has a small pink steam engine in front of it. Been there, forget the name. Lots of stuff there literally in the guy's house, so it's great for "hunt/browse" types.
If you feel like venturing down into MD from Gettysburg there is always Catoctin Mountain Trains in Thurmont. Their reputation speaks for itself. It's a nice drive down 15, too, some nice country out there.
http://www.cmtrains.com/
That pretty much does York and Gettysburg as far as I know off the top of my head and from my own travels.