You should be able to get the networks (ABC, NBC, CBS, PBS, and FOX) from CableONE for "free" (as in, not any more than you're paying now) but you need a QAM-capable tuner. This is different than the kind of tuner you would need to get the channels with an antenna, thus it's different than a lot of the tuners you'd find on eBay. There are standalone tuners out there that are QAM-capable, but I don't think any are cheap by any means.
If your TV has a Cablecard slot, that's probably your cheapest way to do it (just a couple bucks a month), but I don't think any or many "HD ready" TV's have Cablecard slots. Honestly, if you don't already have a QAM tuner, it's probably your best bet to just upgrade and get the HD package from CableONE. You get the tuner, networks in HD, as well as Universal HD, ESPN-HD, and ESPN2-HD, for I believe just $5 more per month if you already have digital cable (it might be $10, I'm not totally clear on the pricing scheme right now). If you don't have digital cable at all, I think it's $20 more per month to get up to a level that includes the basic HD package.