Well, for the prices, there is nothing for you to worry about. As far as I know, the chain sunglasses are not expensive and only cost $13. And it also has some discounts on the online shop. For example, you can buy them on sunglass warehouse. And just link to the website as soon as possible, maybe you have the chance to give the service. http://www.sunglasswarehouse.com/swovvist5.html
And also, the chain sunglasses are very beautiful, so they are popular by many famous people. The chain is attached to the frame and can wear around your neck, or loose in the front. So it can just make you feel comfortable. Just buy them right now.
Sunglasses with chain looks funny and cool. Unlike common sunglasses, sunglasses with chain are not as widely spread as regular sunglasses. I just search it online and find there are some sites provide that sunglasses, and their are sold at cheap prices. Here the links: http://www.claires.com/store/goods/Teens/cat310068/Sunglasses/p57712/Chain-Gain-Shades/
Yes, indeed, at present chain sunglasses are one of the most in things in the world, nobody could resist them I think. As to where to buy, I think they are accessible at every large optical stores actually. You can find almost all types of chains of different prices at some glasses stores. You might as well seek them through online stores, such as Ebay.
You could go to the near eyeglasses store to have a check. Or you could go to the online store to have a search. And the price online is suitable and the quality is just as good as those in the real stores. You could go to the ebay to have a look. The goods there is various.