Asburn is a *terrific* area, there are lots of great people around you. Two come immediately to mind: The O'Connors in The Plains, and Phyllis Dawson in Purcellville. There's also Jo Struby in Middleburg, whom I can recommend personally, and Jimmy Wofford in Upperville. If those names are a little intimidating, I would check out the list of Level I and II certified instructors on the USEA website:
USEA - Directory of Certified Eventing Instructors by Level