We sell a lot of Posh brand clothes for toddlers, they are more tutu type outfits but very adorable. We also sell a lot of dance outfits that could be "fixed up" to look like anything. I would think a solid navy blue leo and a hat or some other head decoration. Maybe some stars or a stripe or two affixed to the leo would be close enough. Something similar to the bigger girls can easily be sewn or made.
Here's some different costumes I found that I would consider if I were looking for the same thing you are:
A flashy tutu style that is rather patriotic looking.
http://www.weissmanscostumes.com/holiday/5052.aspx
This one is a sequined a-line dress style with a leo underneath.
http://www.weissmanscostumes.com/holiday/5144.aspx