Research Papers / What is a good way to find Secondary Sources? [5]

I am having trouble finding secondary sources for my themed Hamlet essay. My essay is on the meaning of life, and my three topics are, that man has an inherent fear of death, fear of afterlife and that the fear causes individuals to desire leaving their mark within the world while they live.

I need to have at least 3 secondary sources, and I would like to have one for at least every paragraph.

Any hints or feedback on how to look or even where to look for these?