Gale: Fine Arts and Music Collection - a great resource for music history.
The Opera Database - you can search for any information you need on any composer from any country in the world.
Academic Search Premier - This is a general resource but provides some music history research.
JSTOR - A good general resource that has a large number of journals dealing with music history and music theory.
ARTSTOR - A good resource on all the arts, including music. This may not have specific music history resources, but it would have manuscripts and scores.
Billboard Magazine - full, searchable access to Billboard magazine's collection from 1894 until the end of 2019.