If you mean cut flowers, the best is probably purple Lisianthus, but you'd need to order those from a florist. Otherwise, for plants, purple pansies. Lavender might, or might not, be in flower in September.

Perovskia...blue/purple

Liriope muscari and asters spring to mind

Bamboo... I just looked up the flowers you suggested and they are beautiful.. my first choice so far. thank you so much.

Andy_jeavon.... what you suggested may also work thank you also.

Andrewr the asters are also pretty which will be in my choices.. thank you

I love the lisianthus flowers the plain white or purple but the white with purple edges are my favorites, they would all look lovely with some gypsophila

If you are planning to buy from a florist it is a good plan to have a discussion with them in plenty of time - often flowers need to be ordered specially. The florist will also know of any flowers likely to be available to the trade but not in season in our gardens. I agree with Pam, the purple and white lisianthus are stunning.