Decide what you want your theme song for: a get-together for a particular group of people who share a certain interest or demographic (teachers, ladies, businessmen, artists, etc.), or a celebration for a holiday or special life event (Halloween, wedding, 40th birthday, etc.). Write down a list of all the songs that come to mind related to these themes and go from there.
With your list of songs in mind, go to a party store and see if its CD compilations include your songs. Party City sells collections of songs themed by celebration types. You can preview these compilations on Party City's website (see Resources), which lists every theme under the sun—from sports, Hollywood and patriotic themes to '50s, movies and girl's night out.
Using the song list you started, think of related keywords you can use to search and download songs from reputable websites. Digital media is so conspicuous and so prolifically published in numerous public forums, that it is often free of charge and so easily accessible that you can find most known theme songs. However, to find songs recorded in good quality and to do it legally, you'll want to purchase songs for download rather than just grab anything you find on the Internet. Services such as iTunes and Rhapsody can be searched via keyword for specific themes. Type in keywords like "wedding," "Halloween," "christmas," "bachelor" and any others related to your interests. Almost as complete as those libraries, and often providing harder-to-find audio files, is Amazon.com. Songs found on Amazon are less expensive and favor MP3s, which are more compatible with PCs than the MP4s found on iTunes. Use YouTube to research ideas. For example, a search of TV theme songs on YouTube will yield videos with all your favorite TV tunes that you sang along with as a child, like "The Addams Family."