Tilda Swinton wears Salvatore Ferragamo FE 2141Q sunglasses in the movie I Am Love. These glasses come in several different color combinations, but the light colored frame as seen in the movie is currently hard to find. Only Smartbuyglasses.com has this exact frame in stock. The black version with gold details is more widely available, for example at Amazon.com for $206, and the stores listed below.
In the same movie, Tilda Swinton also wears another pair which has yet to be identified.
Reference: Salvatore Ferragamo FE 2141Q, top spotted send frame and brown gradient lens, color code 582/13