I have not heard of Calcium supplements being a no-no to IC patients. CA supplements are mostly calcium carbonate along with some additives for the coating. Perhaps, they are referring to the additives on the surface of the pills as no-no.
Vitamin C (a.k.a ascorbic acid) may burn the bladder of IC patients, therefore, I take a buferred form of vitamin C. It is buferred with calcium carbonate. They used sell that form of C at GNC but lately I have been ordering them on the web in large quantities(bottles of 100 or 500 pills 500 mg each). I would not take regular ascorbic acid as vitamin C supplement.