There is more criticism written on authors and works from the past than contemporary authors. Look at it this way: they've been written about far longer!
Well-known authors will have a larger body of criticism to choose from than less well-known authors.
Larger works (novels, plays) attract more critical attention than short stories, essays, or poems.
Don't marry your topic! Have a handful of topics under consideration, do some initial quick searches, see what has a larger body of work, and pick the topic with the most to choose from.
If you have an obscure topic, reference librarians are available to help you!
