The best place to look for these stores are in strip malls. They tend to be located in lower income areas. These stores will not be located nest to upper end stores such as Macy's, and Nordstrums. You can search the web and look in your local telephone directory under discount stores.
Currently, 99 Cent Only stores are only located in California, Texas, Arizona and Nevada. Try looking for Family Dollar stores or the Dollar Tree in your state.
I don't know of a ton that are just 99 cents. Most dollar stores are in poorer suburbs. They are also located in communities that want to have a mix of economic levels. Google it :)