I have this same problem - there's a cup size difference between my right and my left. I've had professional fittings and they size for my measurements and make sure the cup fits the larger side and then told me to stuff the smaller side. I use the sew-in bra cups that you can buy at fabric stores (they sew them into dresses and swimsuits and such), but I think the "chicken cutlet" type that they sell in the lingerie department (and also in fabric stores, too, I think) would work just fine. I like the fabric ones, though, because I can pin or tack them into the bra - they have a tendency to work their way out otherwise.
