HD channels are available as a public service from broadcast stations across the country. To receive digital HD channels (or Standard-Definition digital channels) all you need is a digital television (or digital receiver) and antenna capable of both VHF/UHF to receive the signals.
To find HD channels in your local area you can use the websites AntennaWeb.org or DTV.gov to locate broadcast facilities nearest you by entering your zip code. The results will provide you with directions in which to face your antenna to maximize the signal reception of a specific channel or network.
For more reference on receiving HD channels over-the-air refer to this overview of getting free HD channels.