DA, I have found PAW Pro Biotic to be VERY successful for the rescues and my own delicate little Bella's sensitive tummy.
Paw actually allowed me to purchse some of their Pro Biotic which has a use by date of Jan '10 for less than half price at the vet. I paid $22 per tub and they have assured me that this will be fine (if kept in the fridge) for another couple of years. I purchsed a box of 24 and am happy to send a couple over to you if you wish for the price I paid + postage. PM me if you want some, otherwise, you can purchase at the vet for about $49