Most of the wineries have free tasting (dégustation), without rendez-vous, and sell wine by the bottle. Open hours are often all day, from around 9 AM to 6 or 7 PM in the summer. In some, especially during the off-season, they're usually closed during mid-day, from 12 or 12h30 to 2 or 3 PM. To really appreciate French wines, take a break yourself at mid-day, and sample the wines with the local cuisine.
Guided visits are possible at many of the wineries, but usually by phoning first.
Most of the vineyards we've visited will take orders and ship the wine, but only within France, or sometimes within the European Union.