Most people I've seen to Scar used their own hair are his is buzzed short along the side and the used hair paint in white.
http://www.mehron.com/Liquid_Makeup_p/111.htm
For scar you want to style the hair first with got2B spray then paint over that.
For a wig I recommend this wig in silver as you can reshape the hairline and dye the side with black FW in diluted to a grey tone.
http://arda-wigs.com/products/cady
How to dye.
http://www.cosplay.com/forumdisplay.php?f=256