That left quarter panel is made from steel and can be bent back into its shape. Same thing happened to my Z4 and I was able to repair it myself by punching from the inside by removing the left quarter panel turn signal (bmw logo light)... your quarter panel may need more work to bend back... so you might have to remove it from your car to work on it.
Or... the part is available NEW here for $194 shipped (use free shipping code FREE49)
http://www.jcwhitney.com/replacement...273y2003g346j1