How far this guy is from you: Phillip Dutton Eventing ? I've seen him riding at the big eventing competition in MD and he did lovely lovely test in dressage arena (and just seemed to be a nice person overall). My trainer has a great respect for him.
But overall if you want to learn more of dressage (and you have to anyway since it's one step in eventing) I'd go with dressage instructor, and may be eventing instructor in addition to that or later on.
When I was looking for dressage instructor and later on for eventing one I asked people on local forums for recommendations. Mind you recommendations may be great but the trainer won't work out for you (that's happened to me several times), but I have to say both trainers I work with now I found with the forum people help. Also I may be wrong, but I had a feeling there is a local equine newspaper in PA with trainers and ads (I remember seeing something like that at the PA Expo). May be also a good place to look.
And great recommendation with the equine newspaper. I have plans to see the PA Horse World Expo in February, and I also have a local tack shop that probably carries it. And I didn't even think of local forums!