Winter truffle markets (roughly, from November to March) can be found in towns and villages across Provence.
The four biggest in France (for professionals):
Aups - Thur mornings
Carpentras - Fri mornings
Richerence - Sat mornings; said to be the biggest in Europe, in volume of truffles traded.
Lalbenque (15 km southeast of Cahors [map] - Tue afternoons)
Apt - Sat mornings
Valréas area [map]:
Chamaret (10 km west of Valréas, south of Grignan) - Mon mornings
Grignan - Tue mornings
Montségur - Thur mornings
Nyons - Thur mornings
St Paul-Trois-Châteaux - Mon mornings; and a Truffle Festival on the second Sunday in February.
Taulignan (8 km north of Valréas) - Sat mornings
Valréas - Wed mornings
Rognes - end of December
Périgueux in southwest France [map] has a truffle museum, and markets on Wed and Sat
Nearby Sorges has a truffle market the third Sunday in January, followed by a truffle-omelette meal.
Some Alpes-Maritimes Truffle Markets:
Mid January - Marché de la Truffe, hotel La Bastide St-Antoine, Grasse (a 5 minute walk from the home of Beyond).
Mid January - Marché de la Truffe, Le Rouret (10 km east of Grasse)
Beginning Feb - Villeneuve-Loubet (map - southwest of Nice)
Beginning Feb - Puget-Théniers