The most comprehensive resource for Ken Sugimori's work is the official art book released in by publisher Nintendo Dream. Dogasu's Backpack Primary Art Book: Ken Sugimori Works The full title of the definitive collection is

Ken Sugimori Works: 25 Years of Portfolio from "Quinty" to "Jerry Boy" and "Pocket Monsters"

While official PDF versions are rarely distributed by the publisher, several physical collections and digital archives exist: Ken Sugimori art book works - Book Review

The Cons of Digital:

Without hesitation, I grabbed my backpack and set out on a journey to uncover the secrets of the "lost" Pok茅mon. I traveled to the mountains, forests, and caves, searching for any sign of this elusive creature.