As a popular R&B singer and actor in America , Usher usually wears Ray-Ban, which is very fashionable and be loved by a great number of American super stars , especially rockers. you can buy Ran-Ban in chains shop and it is expensive as it belongs to luxurious goods. The price at least more than on one thousand dollars and be careful to make sure you get the genuine eyeglasses.
Oh. It is looks cool. And i also like his black sunglasses, too. And i actually don't know what types of glasses he wear. But i find some places where sold Usher sunglasses. Check it here: http://www.sunglasses-shop.co.uk/celebritysunglasses/usher.aspx
Actually, the type of glasses Usher used to wear is called aviator sunglasses, especially cube shaped ones. As a matter of fact, such kind of sunglasses could be easily found at some local optical stores, shouldn't be too hard for you. As to the price, you could take it easy, since a lot of normal brands produce that type of sunglasses, so the price could not be that pricey. Generally speaking, 100 dollars would be ok.