Although regular sheet music can be used as source material for this program, the most common source are so called "Fake book", which contain songs showing only chord symbols and melody lines.  Like this:

There are lots of fake books available in music stores and on the web.  To get an appreciation just go to Amazon or Google and type "fake book".    As of this writing Amazon produces more than 650 entries!  Here are just two:

The Hal Leonard Publishing Corporation series of  "The Best Chord Changes" books -- there are five of them, all excellent, search for "the best chord changes" on Amazon.  Each song contains two sets of chords changes, one is from the original song, and another is an alternate harmonization for variation.

Search the table of contents of each of these, and other similar books, to see which of your favorite songs are included.

A typical -- and big -- volume of all styles of music, from Broadway to Country to Beetles.  More that 1000 songs are included in this and other similar volumes.

FastChords is ideal for quickly entering songs and listening how they might sound on a real keyboard.