Forever 21
or Wet Seal

They’ll have a lot more once it’s fall.

Burlingtoncoatfactory.com

I would recommend buying a peacoat in a petite size, that way it is both shorter and smaller. Double breasted jackets always look cute on really petite girls.
I would recommend going to Nordstrom (or something of the sort) and trying on a few that catch your eye, every coat is going to run a bit different so there is no way of telling what cut/brand is going to run small enough to fit you.
I would also recommend black, navy, red, or white- they are timeless, so you can wear the coat for years to come. It will be a good investment.

delias
charlotte russe
wet seal