There are so many glasses shop selling polarized sunglasses. If you want to get it at cheap prices, you can search it online from some reputation online shops who specialty in eyewear. If you want to brands sunglasses, you can go framesdirect.com. If you just look for good quality sunglasses, you can have a look at firmoo.com where a famous online shop provide good quality eyeglasses, sunglasses at cheap prices.
You can just search them in the web broswer like goggle with key words. There are so many online optical stores offering polarized sunglasses. I think online optical stores or web sites appear on the first page are more reliable. You can go to each store to have a look and then compare them with the price, quality and service to figure out one you like best. Try sunglasshut.com or forever21.com which offer polarized sunglasses with good quality yet affordable prices.