In the United States, the only "cable box" you can buy is a device which uses CableCARD such as TiVo, Moxi or windows media center DVRs.
The settops that cable providers lease to you are not sold to end users by either the manufacturer or the cable provider. You can buy them on eBay but they will likely not be usable with your cable provider. If you are buying one on eBay, check with your cable provider first. They will most likely say "no." Even if they say "yes" you need to get one that is compatible with their system. Typically this means that if they're using Motorola you need to get a Motorola or compatible box.
Some providers in Canada (I believe) will sell you the box but it can really only be used on their service.