I use a 42 x 16 set up to commute from Fairlington to Downtown DC. It seems to be well suited to both stop-and-go city sprinting and getting a good cadence on the flat parts of the Mt. Vernon trail.

I've had good success in finding parts at Spokes Etc. in Fairlington, on Quaker Lane. But to be honest, and though it may smack of disloyalty or heresy, I prefer to order my parts online.