This WDC area has a huge represent for APs.
I am with APIA, but I have friends that are with APC (au pair care) and they have big clusters, too.
CC has a Facebook group for the Mid-Atlantic.
I am pretty sure they have big clusters' around here.
https://www.facebook.com/groups/20014578926/?fref=ts
You might find smaller clusters with the smaller agencies (GoAuPair, EuroAuPair, etc.)...but since this is a metro area, they will be bigger than more rural areas.