The largest professional organization in the world dedicated to QS and Cost Engineers has just got to be the Association for the Advancement of Cost Engineers, International (AACEi.)
http://www.aacei.org/ Here is a blurb that I ’stold’ off their website,
Since 1956, AACE® International has been the leading-edge professional society for cost estimators, cost engineers, schedulers project managers, and project control specialists. With over 7,000 members worldwide, AACE® International is the largest organization serving the entire spectrum of cost management professionals. AACE® International is industry independent, and has members in 80 countries and 80 local sections.
I am a member and I highly recommend it!