Both houses have been rented
Links that may be of interest:
CraigsList - Washington DC rental houses and apartments - New York City - and many others
Zillow is actually decent for finding rentals (it is really focused on buying real estate but an increasing number of rentals are shown. The numbers are limited but the details and usability of the site are good.
Washington DC Summer Housing Guide
Places 4 Rent
Washington Post Rental classifieds