I used Volta Electric to install my 14-50:
Electrician | Electrical Contractor | Arlington, VA | Volta Electric
I'm in Fairfax, but they say they serve your area too. Total cost was a bit over $400 for a short run, which is kind of high, but it was about half of what the Tesla-recommended electrician wanted, and this is an expensive area. They were friendly and did good work (as far as I can tell, anyway).
By the way, you may want to repost this (or ask a mod to move it) in the Mid-Atlantic section, where you'll get people in the vague area:
Mid-Atlantic