You can get OTA tuners on ebay for $100 or so. I've also heard people get DirecTV HD receiver at places like best buy for $75 w/ an open box sale. I've never been that lucky. I even considered getting an HD OTA tuner for my parents since they don't watch cable. W/ HD OTA, they'd at least get a clean picture w/ the channels they do watch.
If you get HD via cable or satellite, they will provide you with an appropriate HD set top box.
Alternately, you may want to look at new TV's/plasmas/DLP/LCD etc that support the CableCard format. W/ this, you tell your cable company to provide you w/ a cablecard tuner. You plug it into your TV and you're set. That's a slick solution, but it's just starting to come you. My understanding is that cable providers are required by law to support this. Tivo has already announced a Tivo-capable cable card by the end of the year.
Time Warner
CNET 
