I've used D&B Tailors (Newtown Square). Exceptional for alterations - sends out "custom" suits and coats to Gilibertos but very precise with final products. He's the best I've found - not cheap but very reliable and good.
I have used a tailor (the guys on the showroom floor - not sure if these are the same guys that do actually tailoring down below) at Boyds and asked them to do some tailoring on the side. His name was Gainni Russo.