I don't shop at Cosworx, but Amphigory has a wig that might work for what you need. This one.
You can easily get the curls out (just put the wig in hot water and let dry until straight). I've had great success with styling Amphigory wigs, and it comes in a bright yellow similar to that girl's hair. You can also look around more on there, but you can stick with cosworx if you like.