cuiremk-
Tarbais beans are white beans, though I've never cooked with them. You could probably substitute another type of white bean. I'd check out Treasure Island and then Fox& Obel for both products (F&O being the pricier option). I am often surprised by the recherche little delights TI carries. In the past, I have bought goose fat at F&O, but GWiv said they were out when he went there, so call ahead to see if they also carry duck fat. The confit I have definitely seen at F&O.
P.S. Check out the events calendar for an upcoming cassoulet lunch with Chicagourmets after the Culinary Historian's bistro talk-- you might want to try another local version. Kudos to you for seeking out the best ingredients. Might I suggest a touch of Armagnac as a digestif after your meal? With cassoulet, your'e gonna need it.
Man : I can't understand how a poet like you can eat that stuff.
T. S. Eliot: Ah, but you're not a poet.