Source? Let's hear it for Virginia!!
But these numbers don't surprise me, unfortunately. Nor does the fact that Episcopal churches are concentrated in the East and especially New England.
If there's data for Congregationalists, I'd assume they're almost completely exclusive to New England.
Weirdly enough I've twice gone primarily to a church that meets in a Congregationalist church (and two different ones) including my current one. But I've never attended an actual Congregationalist service unless you count our joint Good Friday one a few years ago.
But Amy Klobuchar is a Congregationalist so they're not exactly foreign to Minnesota.