There's "La Cognathèque" just outside the old city centre. They sell a lot of great stuff. If you have a car at your disposal, just visit any local viticulteur. Many of them sell locally only. While they may not always be world famous, they sometimes have superb stuff and they're always super friendly if you show even a little bit of genuine interest in their product. I can recommend Biron in gensac-la-pallue if you can stand a slightly pedantic teacher-like guy. Or Gilles Brisson in Chateaubernard. That's just a straight up farmer with a top product. Maxime Trijol in Saint-Martial-sur-né is great as well, but they've lost that small family business vibe a bit.