There are many ways to find primary sources via the library or the web. Use whichever method works best for you!
Use Advanced Search. Pair your composer's name with phrases associated with primary sources, like letters.
And you'll find books that have compiled and published primary sources like this:
The Bach Reader: A Life of Johann Sebastian Bach in Letters and Documents by Other good primary source keywords include:
Simply search Google for your composer's name and primary source keywords.
To limit to educational websites, add site:.edu to your search