I found one on ebay. It took some looking to get an all-white, pretty, not sleeveless for the price I wanted. I just checked, and the one I bought was still there. It shows a head piece, but does not come with it, - unless that has changed. The flower on the waist was removable, and the dress is machine washable. The sizes seemed to be accurate. I bought slightly big, just in case, and on my daughter, it was slightly big, but looked great.
seller = kidstudio1908
item = T6 white flower girl dress communion Sz 2,4,6,8,10
After you buy, you tell them what size. I did the "best offer" and offered $15 and they accepted, so it was $25 with shipping. You could even try lower and see what they say...
There are a lot of other styles on ebay, too.
Good luck!