Where is the best tunes

Ask your dancers to bring in their favorite tunes, on CDs or music files.  Don't play them blind, because many aren't danceable (see the above criteria), but collect them for your future programming.

Search through Spotify, iTunes or Amazon mp3s.  If you buy tunes, you can deduct them as business expenses if you're a professional DJ or teacher.

Many complete tunes are now posted on YouTube, often with just a single image of the cover art as the visual.  Then there are several online sites that convert the YouTube videos to mp3s, and a hack using Safari, giving you the entire song for free, but the monaural sound is often too low quality for a dance.

Try the listening stations at record/CD stores, before they completely disappear (oops, too late).

Trade favorite tracks with other dance DJs.

Listen to the radio, including Pandora and internet stations.  Stations often post their playlists.  Internet and satellite stations (like Sirius XM) also give metadata of the current tune.

This website