Cable, in the United States, uses QAM modulation (which is a lot more efficient than
ATSC's
8VSB modulation, but would be more susceptible to interference if used
OTA). So you need a QAM tuner.
There were a few stand-alone QAM tuners for sale a couple of years ago, but they ran $180, and no one bought them, so they've been discontinued. Now, mostly QAM tuners are found in DVRs, built in to
new televisions, PCI cards and USB add-ons for computers, and in equipment that the cable company rents. (The cable company has no choice; they have to buy these tuners, because they're required by law to make them available to you for rental.)
In my search this morning, I could not readily find a stand-alone (non-
DVR) QAM tuner for you to purchase. Sorry. There just isn't a big enough market for them.
Do keep in mind that if you do find one of the old ones on eBay, it still might not do the trick for you. All it would be good for is converting in-the-clear QAM. If your cable company encrypts channels, to preclude cable theft, then you'd not be able to receive those channels with your own, stand-along QAM tuner. You'd need either their cable box, or your own host device that is CableCARD capable (and there has never been a stand-alone QAM tuner with that feature; there has never been a big enough market for that).