Here is a tailor in the Sanlitun Soho complex (basement of Plaza Building 5) I used for alterations only, but came highly recommended from local expat friends (and his name card says he's done work for high level Chinese celebs etc, so take that as you will). Work came back good, was fast, and reasonable price.

Kevin Lee
李健军

Soho Plaza 5-B1-530
Sanlitun Chayang District Beijing
+86-136-8336-3913
email: [email protected]
Hours: 11-8pm

Uncertain of English ability...but his name card is half in English.