Finding a flower girl dress depends on if you are having a very casual or formal wedding. For a casual wedding, shopping chains like jcpennys, kohls, belks etc. Can offer stylish dresses at reasonable prices, and even some second-hand shops can have gorgeous dresses for cheap. If you are looking for something more formal, try a bridal shop because a lot of big chains (and even some local shops) will carry not only wedding dresses, but bridesmaid dresses, flower girl dresses and even mother of the bride/groom dresses.
It's not that I know anything about flowery dresses, I just want to point out that you are asking this question to the whole world, so to speak. People from all over the world log in here. Some girl in Ivory Coast may know someplace but it may be too far for you. So, telling us where you are located, roughly, might help.
Because small children grow rapidly I wouldn't actually buy a dress until close to the wedding day. I would try looking at local places that specialize in communion and other formal occasions for children. Call or visit these locations and find out if they have a good selection of ready to purchase dresses or if you need to order in advance. I also would make sure the little girl is comfortable and the material isn't itchy or hot as this will cut back on her fidgeting during the ceremonies.
I love the simple flower girl dresses on jcrew.com, but they are a bit pricy for just a tiny little girl. Honestly, it might be a good idea just to head to your nearest department store and rummage through little girl dresses there. Once something is specifically labelled "wedding," the price jumps ridiculously! You might be better off shopping at a department store and looking at their regular selection of little girl dresses than at a specialty bridal shop, which will just inflate prices.
Another hint? Do some searches on etsy.com. I'm sure you could find an adorable handmade dress for not a lot of dinero. :)
For my wedding, I went to rk bridal. They were very attentive, full service, much less expensive than their competitors. They have a wider selection in their store than they do online, if you happen to be in new york. Their tailoring is also a lot less expensive and they do a great job.
Try 'mysweetdress.com'. I bought two flower girl dresses for my daughters and I spent much less money than bridal shops. They have similar dresses as dresses I can see on US Angel website, but in much less prices! Or, you can search ebay. They have tons of cheap flower girl dresses.
I can highly recommend david's bridal for flower girl dresses. They have actual stores throughout the country and online. Just like their bridal collections, the flower girl dresses range from simple and cute to elegant. The service is great and they even have a return policy if ordered online. You can get the style code you like online and order it at your local store.
A fantastic selection of flower girl dresses are found easily from the internet from around $30. 00 and up. Of course many cities have women's second hand shop where these items are bought and worn once, selling for around 60% off. The same with bridesmaid dresses, wear them one time, and get 70% off original price.