There are many on-line vendors, and there may be a store near you that carries a variety of video cards. Why don't you try a google search for the the models you want to research, for example, "shop for HD 4850"
I definitely also recommend Newegg. A lot of their video cards have free shipping, or low shipping costs, and the prices are pretty much some of the lowest you will find anywhere. There is also a wide variety of cards to choose from (as of checking a minute ago, there are 10+ 4850s listed).