Remember to look for writings by the composer and for newspaper articles that were published around the date of the composition's first performance.
Some databases you might use include:
Google can be useful if you construct your search carefully. Try searching for the composer's name, a word from the title, and "program notes."
You can also read many examples of program notes using Naxos. When you find a CD, look in the left sidebar for a Booklet link with a PDF symbol. These liner notes are usually find on the Naxos label, but some others have it as well.