This is a common question and not always an easy one. You see each cable company has to sign an agreement with each local affiliate in order to make this happen. Some affiliates are owned by large media companies and can refuse to play nice. A perfect example of this is the Sinclair Broadcast Group. Sinclair is a bad word on many HD forums and for good reason. To our knowledge they have not signed one agreement to have their HD channel carried anywhere in the country. At one point they were actually asking cable companies to pay for their HD signal which is unheard of. This is really bad considering they own 56 stations in 36 markets and unfortunately for .aCr. San Antonio is one of those markets.
The only thing people in these markets can do is to setup a OTA antenna to pick up the HD version of these channels.
Does Sinclair have a station in your market and is it carried by your local cable or satellite company?