How did Jon Krakauer find all this information for him to write Into the Wild?

Non-fiction of such excellence is only possible through a
considerable amount of hard work and effort. The research that it takes to investigate
such a story occupies many hours and involves much diligence on the author's part. You
might like to consider the "Acknowledgments" section of the book at the very end,
following the final chapter, where Jon Krakauer notes down a series of people that shows
the extent of his research and the involvement of others in his work. Note how the first
paragraph of this section of the novel makes the debt that Krakauer owes to the family
of Chris McCandless evident:

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Writing this book would have been impossible
without considerable assistance from the McCandless family. I am deeply indebted to Walt
McCandless, Billie McCandless, Carine McCandless, Sam McCandless, and Shelly McCandless
Garcia. They gave me full access to Chris's papers, letters, and photographs and talked
with me at great length.

This
of course shows one of the major sources of research for this work, but at the same time
we can also infer that considerable time was spent interviewing other characters that
appear in this book and also researching the flora and fauna and literature that is so
important for this tale.