You can go to AT&T store and get one free if you have a poor coverage at home and are not selected. It is quite easy if you have a family plan, are on AT&T for a long time, or have other services with AT&T such as DSL and cable.
http://www.engadget.com/2011/01/21/atandt-offering-free-microcells-to-top-7-5-percent-of-customers-l/