From what you said, I can see that you are looking for a good place to find sunglasses at a cheap price. So, indeed at some large optical shops the prices for sunglasses could be bleeding, and we would try to look for other places, such as online stores, here I would like to recommend Amazon and Alibaba to you, where you could see loads and loads of different sunglasses of good quality. However, some of them are not famous brands.
The sunglasses sold online are much cheaper than that sold in any eyeglasses stores in your local area. And it is said that often they are 80% less than that sold in physical stores. But I suggest you to have a look at the physical stores before placing an order online. So you can try on the sunglasses that you like and have a good understanding of whether the sunglasses that you choose are fit for your face shape or not.
The good sunglasses that sold in physical stores are very expensive. If you want to spend less money to buy a pair of qualified sunglasses, you should purchase them when there are some activities or celebrations hold by those physical stores. Usually they will give discounts to give back to their community and customers. Hope this helped!
Those sunglasses are very cheap. You can have a look at http://www.firmoo.com/wrap-around-rx-sunglasses.html But maybe they are not the cheapest kind when compared with other sunglasses sold by other online stores. However, they can guarantee that the sunglasses can protect your eyes from the UV rays which some cheap sunglasses can't provide you with this kind of services. Hope you will like some of them.