I'm from the South of France and I've been addicted to cassoulet for the last 45 years.
So believe me when I second Geo's and Michael's opinion: A good quality kielbassa will do the job.
Check Paulina's Market on Lincoln Avenue, they used to make a decent one.
But I agree that the Saucisse de Toulouse from Fabrique Delices in Hayward, CA is as close to the French original that you will find in the U.S.
May I suggest that you follow Ann Willan's recipe from her marvelous book French Regional Cooking.
But it is for a Cassoulet Toulousain, not for the Caassoulet de Castelnaudary that I prefer.
But they also have several good recipes on Epicurious.com
Good luck
Alain