In Bowie Maryland: http://www.baysidelimo.com/limo_rentals/n_rolls.php

In Baltimore: http://www.catonlimo.com/vintage.htm

DC area: http://www.regallimousine.com/ (Bentleys and Rolls)

There are quite a few more... The trick in Google was not to include the word "rent" but just "rolls royce wedding washington dc" or alternately "classic car wedding washington dc"