A cookie is a foam or silicone pad that you can insert into any bra see the top two products pictured on the right. A few things that jumped out at me.
Most convertible bras will convert to a halter or racer back position. There are a host of push up bras available that can enhance your bust AND fit you well see the top two bra styles featured on the right. Natori is fantastic for this, and they do have a styles in A-C cups.
Semi-full breasts will not have quite as much breast fullness when wearing a strapless bra as a woman with full upper breasts. The inventory changes due to the supply.