That's, ah, kind of retarded that the modem is a line-item on the bill - why line-item it, when you can't buy one?
Generally, yes, they're supposed to line-item things - but, this is the one thing that (AFAIK) they aren't required to, as it's not something you can buy, nor can it be part of another piece of equipment (like, say, a CableCARD can be.)
AFAIK, there are only two companies that even make residential cable/VoIP modems: Arris
, and Cisco
Now, if you can find somewhere that will (legitimately) sell you one, and Comcast is willing to provision it, go ahead by all means.
But, as I said earlier, they generally aren't sold to the public - only to MSOs (cable companies.)
How "not sold" are they? Straight from Cisco (from the DPC6027's page): "Available from select North American service providers. To check availability in your area, contact your local service provider:"