@Bonedome,
Dunno when you'll be arriving but we allways arrive wednesday evening or thursdaymorning which leaves ample time to do your shopping. There are many big supermarkets situated in and around LM and Arnage.
Friday is by far the worst day to go in your car and to the supermarkets,since everybody does that and you've also got the majority of the french coming on Friday.(if you still go,don't be surprised to spend 4 hours away from the track/campsite)
We mostly go to the supermarkets once on thursday and once on the saturday,about two hours after the start of the race.
Hope you can use this info.