For general food shopping, there is small Casino Supermarket with everything you need. The big Intermarche on the way out of Praz towards Megeve has more choice, including good vegetables and fruit.
For meat, there is a wonderful butcher in Flumet. Try the "Filet de Porc" trimmed in French style so it has just enough fat to make it moist but not greasy , or the Diots sausages (ask for the easy recipe card for Diots au Vin Blanc). The same family own the excellent cheese shop a little further up the road. If you want a fondue, they will lend the set including fuel - just reserve it in the morning, and tell them how many people you are feeding. They also sell their own charcuterie and will slice it for you.
There's a bakery and Patisserie at the top of the street in Flumet on the left, or if you want a fancier Parisian style bakery, try the one on the way our of Praz (as you head towards Megeve) on the left hand side.
For easygoing, inexpensive eating out try the "Resto" Cafe-Restaurant in Flumet just up from the cheese shop. They have an amazing terrace overlooking the gorge and serve standard local fare such as fondue and steak and chips. For a sophisticated, but good value meal, try the lovely chalet la Source in Saint Nicholas la Chapelle (it's the place with blue shutters). The walk up there by the back road from the chalet is beautiful.
Further afield, Maryse's pizza past Notre Dame de Bellecombe is lovely. Chez Therese up on the Gateau hill on the opposite side of Flumet is fashionable. Of course, if you want a real night out with no expense spared, to Megeve has some famous places to eat such as Flocon de Sel and the Alpaga - make sure to book long in advance.
For wine, the best place to go is probably the Nicolas in Megeve (near the tourist office) which has an excellent selection, reasonable prices, and offer practical advice if you tell them what you like. The butcher's/cheese shop in Flumet has well selected local wines. The choice in the supermarkets is not inspiring, but the Intermarche in Praz has a display at the end of the aisle, with wines that have won in competitions and are usually reliable.