Comcast og-to-Digital Converter Box Advice?

I need an extra ogue-to-digital converter box for an extra Comcast wall outlet in a third room. Comcast wants to charge me a lot in perpetual monthly rental fees for such a box. Can I buy one elsewhere (like on eBay) that would be compatible with Comcast's proprietary signal type/algorithms? What will work and what's the most inexpensive route, given that this third outlet just services a small extra TV for the kids?