Many women in the bra fitting community have frustrations with being in between sizes. I myself have found it extremely frustrating to not know whether I should be wearing a 30C or a 30D. I can fit into a 28DD, 30C, or 30D depending on the brand and style. However, I think that when I wear a 30 band I am really a 30C½. I just went to ThirdLove and discovered that they carry half-size bras! They come in fun colours and three styles (Plunge, Balconette, and T-Shirt).
Here is their size chart. As you can see, they have an assortment of sizes not commonly carried by other bra brands.