Swiss pass Chamonix

Greetings! I am traveling to Chamonix from Geneva airport in July.I will be then traveling around Switzerland for 8 days[after hiking around Mount Blanc] Can I buy a Swiss pass from Chamonix? If so,where abouts in Chamonix? I won't want to "validate" the pass until I have completed my hike. I presume my 1st day/trip could be from Chamonix Mount Blanc, to Brig?
The reason I prefer not to buy a pass [online] before I leave home is I may not have sufficient time to workout and cost my travel plans in sufficient time to order+receive pass before I leave.
PS "What a well run/informative site....I will be visiting+messaging again!":-)

Hi
You can buy your pass in Geneva but have the agent there put the first date you will use the pass on it. Not the date you bought it.
Dick