As someone who searched high & low to find the perfect Rogue (X-Men) costume, I can tell you that an off-the-shelf version does not exist. I had my "classic" Rogue (green & yellow) costume made by BlackCerenity.com and it's awesome. However, the hair was another problem. No wigs for Rogue's unique white streaked on brown style exist either; besides, I'm not that fond of wigs as they can be very hot and sweaty which makes life in costume uncomfortable.
If you are committed to dying your hair for your character's perfect look, check out some hair extensions. There are rather complicated ones that are hot glued into your hair by a professional or there are clip-in versions. I was very excited to find a sale on Easilites today. I wish I had seen these back in October. This brand has all the basic hair colors in natural shades but they also have every punky color available too. You pick two colors for your clip and get a blended look. So for Rogue's white streak, since my hair is not dark brown, I picked a honey blonde color to blend with the White Chocolate. The honey color will blend closer to my real hair color (hopefully) and show off the bright white to get the streak. Since there's a sale going on, I also ordered some Blueberry and Pink. Can't wait to see those!
The best prices I found were at Best Wig Outlet.
[Edit]: I received my blue and white (really platinum blonde but close enough) extensions. Once I figured out how the heck to undo the clips I was fine. It takes some practice. And because the clips are obvious, you can only use them for areas where you'll pull more hair over the extensions to hide the clips. It gives that same kind of cool look that Avril Lavigne, Kelly Clarkson and Christina have used for a fraction of the price and more temporary then glued-in extensions that last up to six months.