I highly recommend Arda wigs. They are the only brand I buy from now, since I can always count on the quality of the wigs and their impeccable customer service. (Lol, no, I don't work for them or anything. I am simply a very impressed customer.)
You should check out their "Ferrari" wig in "Serah Pink", a color that they especially blended for the character. The wig is of high quality and there is a lot of fiber, so, the side-poney is thick and luscious. The Ferrari wig is already curled, so all you really need to do is cut the bangs and the back of the wig, and put the rest in the a ponytail.
My sister cosplayed Serah last year, and the wig was the highlight of the costume.