Kate wore this plaid skirt and black turtleneck/blazer outfit in London back in 2008. (Photo credit)
If you want to recreate Kate's classic preppy look, I found all of the elements you'll need. Click on any of the images below to purchase - an alternative skirt + price information can be found below the photo.
Blazer - $29.80
Turtleneck - $12.99
Skirt - $48 (click here for an alternative option for $20)
Tights - $6.50
Boots - $49.99 (on sale)
Bag - $24.99