These boxes aren't legitimately available for sale in the US. They are only available through your local cable provider. If you purchase a used unit it won't work because its unique id number will have already been registered in another cable system elsewhere. I would suggest either obtaining a box through your local cable company.
Thanks Stephen. I called my local cable company and they said that they would add their own programing and chip to the box free of charge so I guess it depends on which cable company you deal with.