Which part of tignes? We were in val claret. The supermarket was pretty well stocked although small. I didn't particularly notice fresh cow's milk, but they had quite a large fridge section with soya milk and yoghurts, so I'd imagine there was cow's milk.
My previous experience in Val Claret was that it was never a problem. Took ds when he was small (admittedly the last time we went was a 5 years ago) but the supermarket always had fresh milk - we were certainly never without it.
It isn't usually a problem (look for lait pasteurisé) which usually comes in bottle with a blue top if you like semi skimmed (demi ecremé). Sometimes our supermarket runs out, so don't leave it to the last minute!