Challes will indeed be the closest gliding club in France, and Aosta in Italy. From Chamonix, you could also consider Bex or Sion in Switzerland.

Bex (Switzerland) : http://www.lesmartinets.org/2013/?q=node/11
Sion (Switzerland): http://www.vvcvalais.ch/site/pratique
Aosta (Italy): http://www.aecaosta.it/ITA.htm