You can purchase a used Samsung Galaxy S4 from Amazon.com. There are several options from which to choose, allowing you to choose color and carrier. If you don't know which service provider to go with, such as AT&T or Verizon, purchase an unlcoked phone compatible with all carriers.
From the Amazon.com home page, type “used Samsung Galaxy S4” in the search field located in the header of the web page. As this gives you a list of options for that specific model, both new and used, make certain to click the link for the used options for the phone you wish to purchase.