“YES you can buy your own box... BUT [Comcast will] tell you no that it is illegal and that you must disconnect it from their line.” Tom's Guide Forum · 13 years ago
While buying a TV adapter is difficult, you can buy your own compatible cable modem to save on monthly rental fees. What's the difference between a TV Box and a TV Adapter? can i buy a digital adapter for comcast
Users on community forums often warn that while the hardware is physically available, it is frequently a "paperweight" without official service authorization. “YES you can buy your own box