Sky Acres Airport is probably the closest one to Dover Plains; it's in Union Vale and is listed as a "reliever airport for Westchester County Airport." I have been under the impression that it is a private-use airport and not a commercial one, however.
The closest airport that flies commercially is Danbury Municipal Airport but its options are very limited. The nearest international airport is Stewart International in Newburgh, which is probably at least an hour away from Dover Plains. This one used to fly quite a few places commercially but nowadays has its limits as well. The big airports in New York City are about two hours away from here but your options are unlimited there.
