I've swapped out my 20 lb cylinder at the Manassas Roberts, cost me $22, easy as pie
Pick up some spare gaskets for the regulator while your there, you never know when you'll need a new one
There are Roberts in:
2929 Eskridge Rd
Fairfax, VA 22031-2213, US
45000 Underwood Ln
Sterling, VA 20166-2305, US
8607 Quarry Rd
Manassas, VA 20110-5348, US
HTH
Anton