I'm looking for a dark green bridesmaid dress. It's for my niece who is 9...

Jjshouse.co.uk do junior bridesmaid dresses

My niece was able to fit i to a size 8/10 you may need to just shorted the length

I ordered mine from for her and for him, made to measure and they are really lovely and good quality. They do some beautiful shades of green too x

Have you looked at Light in the Box? Go slightly larger on size...

https://m.lightinthebox.com/c/junior-bridesmaid-dresses_1987?_ga=2.34901281.1284016686.1616057791-1268776247.1616057791