Find a Threading Specialist
Look out for good hygiene (therapist should wash hands before touching your face) and a client consultation before you have your first session, including aftercare advice. After all, you don’t want to have threading done if you are having microdermabrasion or a facial the next day.
www.babtac.com (ignore the bit that says there aren’t many therapists who do threading, they have loads of listings)
www.beautyguild.com
www.beautysurvival.com/threading.html
www.embodyforyou.com
If you are in the US then you’ll find local listings here:
http://www.beautysurvival.com/threading.html
Plus you may find listings at Craig’s List too.