The creator of a musical score is the composer. Most online web resources for this material such as those found on music-scores.com focus on composers of the baroque era like Bach or those of the classical era such as Beethoven or Mozart.
According to theaterhistory.com, the idea of a musical play as a distinct, artistic genre took shape in America in the 18th century. This form came to full fruition with music scores for plays produced by composer/writers such as Richard Rogers (music) and Oscar Hammerstein (lyrics).
The rock-era of musical score composition was inaugurated with Andrew Lloyd Weber's (music) and Tim Rice's (lyrics) creation of the rock opera, "Jesus Christ Superstar." More recently, many rock composers and creators of other contemporary musical forms like Elton John, Danny Elfman and even rap artist Eminen have produced soundtracks for movies.
The piano, flute, clarinet and violin traditionally are the most popular instruments used to perform musical scores according to music-scores.com. But, the contemporary era boasts use of electronic, digital and synthetic instrumentation.
Internet downloads of traditional music are plentiful. Most of this music is in the public domain so copyright protection has expired. This is not the case for contemporary compositions. Generally, free downloads of these are illegal.