Google the Wiki article (or related articles), then scroll down to the bibliography. Find entries that look relevant and their authors, then use scihub to find the full books/papers. Have a read of a few, and find around 3-4 ''primary'' historians you feel provide the best answer to your essay question (but also provide a range of interpretations).
Also, begin by writing what YOU believe were the reasons the Roman empire fell, then try and find historians that support/oppose your specific point(s). You only need to write a few sentences, so be really precise in articulating what YOU think. There is always bound to be historians that support your point, unless you write something very out there!