I suspect that this information is available at the Census Bureau. It seems like something they ought to have. I spent some amount of time there, but can't locate exactly where they keep it.
I found this page: https://www.census.gov/geo/maps-data/data/kml/kml_tracts.html
But when I download the data for VA there's no mention of Lorton in it. Nor can I find any code associated with Lorton.
This is just one example. I want to know how to get KMLs in general for different areas. So a general solution would be preferable to a specific answer to this question.