If you look for small bra size you must wear bras with cup sizes A, AA or AAA. B cups are also considered small. However, some visitors look for petite bras. Petite does mean small, but in fashion world it usually refers to women who are 5'4 tall or under.
Don't worry though, I understand what you mean by petite when searching for bras. You mean small band size.
Just for your information, the smallest band sizes I found so far is size 28 bras. And they can be of almost any cup sizes.
Let's come back to small size bras, i.e. small cup bras. Where can we find them?
Here are the stores that carry small size bras and I put them down in alpabetical order.Amazon Intimates is a great place to easily find any petite lingerie by narrowing down your search on the left column of their site. They also have the smallest AAA cups and 28 band size bras. They also carry all styles, as well as popular petite sexy lingerie.
Debenhams lingerie section offers bras starting from A and AA cups. And again they also have smallest band size 28. They quite often do multiple packs. You can definitely find your first bra right there.
Another place to look for your bras is LulaLu. They specialize exclusively in petite intimates. Their bras cost over $40, some styles you can get for less if sale is on.
HerRoom specializes exclusively in lingerie. You can find any sizes and styles there. It's very easy to navigate and find any lingerie that you are looking for.
The little bra company is the company that sells little bras in A cups from 28 to 38 band sizes. Their B cups come in sizes 28 to 36. And you'll be looking to spend about 50 USD and more.