Try Lazy S's Farm Nursery - I haven't bought from them yet but they seem to have a nice variety and good prices:
There are two trees that go by the common name lemon bottle brush, Annette. One, Callistemon pallidus, has yellow-white flowers. But Callistemon citrinus is also known as lemon bottle brush because of the lemony aroma of the leaves. It, however, has the traditional red flower. If you turn to an online source, which you may need to for such a specialized plant, make sure you're getting what you want.