In reply to TomAlford:
Probably repeating what others have said...
Big intermarche on junction 21 on the motorway is where we went for our 'big' shop. It's open on Sunday's until 12.30z
Lidl in sconzier nearish decathlon.
The wee superu in Argentiere was actually ok to, like a tardis and surprisingly well stocked.
Avoid the casino chain in Chamonix unless desperate.
For gear , unless really specialised then there is a huge Quechua store in domancy (branded mountain store) just off the motorway.