I almost always get the "Boudin Basque" at La Fontaine de Mars--I love it. The boudin noir is taken out of it's casing and is served delicately mounded on a bed of slightly sweet caramelized apples. The boudin is consistently perfectly served, slightly crisp on the outer edges and delectable towards the center.
For more info on the restaurant itself check out this recent thread: http://chowhound.chow.com/topics/793337
More places serve this in Paris than can be mentioned.
"Le Verre Volé" (rue de Lancry 10e) always has it on the menu (they get it from Rodolphe Paquin). You can also almost always get some at "L'Avant Comptoir" (Odéon, 6e) (made by Philippe Camdeborde in the Béarn). Both are good.
Bon appétit !
L'Bistrot de la Universite at 40 Rue de la Universite around the corner from Deyrolle has it always. Do remember the French version is vastly different, and IMHO better, than either blood pudding, the English version, or morcilla, the Spanish version. Smooth as silk and very gentle some are flavored with interesting spice notes, as cinnamon or mace. Most butchers have their own version and are easy to cook at home, only note is to cook very slowly as the skin bursts if too high heat. Traditionally served with sliced apples, also great with confit of fennel.