I'd try contacting any really high-end shop -- from Saks on up.
In my personal, pre-Internet experience, I bought a too-long dress at Nordstrom's and their in-house seamstress hemmed it for me -- impeccably. This was a few years back but perhaps if you telephone the store, you can talk to that department directly and ask if their people take outside work.
Also, I've had both competent and incompetent tailoring done by dry cleaners. And I had one really disastrous experience, with a neighbor who claimed to be an expert ...
Or, if you try Googling seamstress DC you'll find several, with reviews (mostly from brides-to-be).