Quimper and Vannes have all the big French High Street names and a lot more besides - patisseries and chocolatiers spring to mind. Both have a warren of medieval streets for centres, well worth a visit even without shopping to do. For a spot of luxury, try Pont Aven with its many galleries and the designer interiors shop Idées. Or you might like the artist studios in Pont-Scorff. Even in more rustique nearby Quimperlé, you'll find all sorts hidden away.
Around here you could explore a different market every day of the week. For the less ambitious, market day in Quimperlé is a Friday, and there is a large market in Quimper on a Saturday selling just about everything. There are also covered markets in both places open daily selling gorgeous fresh produce - Les Halles in Quimper is especially wonderful. For fish markets, you might also like Concarneau or Guilvinec.
For when you just can't be bothered, you can buy the popular plat du jour at the farm shop in Querrien village to heat up later at home, or get fantastic takeaway pizza from the sports bar in Lanvénégen, or if you're lucky, the mysterious pizza van of Saint-Thurien.
There are large supermarkets in Quimperlé, Bannalec and Le Faouët where you can get nearly everything (including the cheapest fuel), and for things like milk, use one of the village shops. These are closed on Mondays and of course for long lunches, but are open during the rest of the week till about 7pm and on Sunday mornings, when everyone comes out to buy cakes from the boulangeries.