Viognier is not as difficult to find as it once was. Condrieu and Château Grillet are the only appellations that use this grape exclusively. Great Condrieu producers include Cuilleron, Gangloff, Guigal and Delas.
For producers of wines made exclusively from Roussanne or Marsanne, it becomes a little more challenging. However, here are two good leads: Domaine Pierre Gaillard for 100% Roussanne and Chapoutier for 100% Marsanne.
The houses that blend Roussanne and Marsanne to produce dry whites are the white wines of Hermitage, Crozes-Hermitage, St-Joseph, and St-Péray.