They are also available in the Rashomon book edited by Donald Richie, and which is a collection of critical essays on the movie as well as including a continuity script.

If you happen to have spare time on your hands, reading Ryunosuke Akutagawa’s short stories “Rashomon” and “In a Grove” might give you an insight into how the original stories were adapted to film. Both stories are available in most Akutagawa collections: see for example Amazon.com or Amazon.co.uk.

The stories are also reprinted in the Criterion DVD.