I'd probably start by calling some high-end, trustworthy boutiques in the area and ask where they get their tailored clothes altered (or who they recommend in the area). Something like Saks or Barney's. A place where they sell expensive suits (even if you don't have tailored clothing, someone who can alter a suit jacket can slim down a shirt).
Then I would just send a cheap garment to them first to make sure I like the work. If I do, I'd bring more expensive/ complicated jobs.