So many sources! Remember-Baum himself wrote 14 Oz books All told, there are 43 "official" Oz books, plus hundreds of non-canonical works. If you want some non-traditional concepts (aka-anything that MGM doesn't have under lock and key), there are quite a few areas to look at.
1. The original illustrations of John R. Neill. While W.W. Denslow was the illustrator of the original Wizard of Oz, Neill took over on the second book, and remained with the series until well after Baum's death, including writing a few books of his own. Neill's work really influenced the look of "Return To Oz," the 1985 Disney live action film that alienated lovers of the MGM musical, but mostly delighted Ozzites like me.
2. Eric Shanower. Comic book artist extraordinaire. He published a series of Oz graphic novels- (all titles in format of " The <insert name> of Oz"): Enchanted Apples, Secret Island, Ice King, Forgotten Forest, Blue Witch. His look is sumptuous, and fairly modern.
3. Skottie Young. Another comics illustrator. He teamed with Shanower from '09 to '14 and adapted Baum's first 6 books. They were published by Marvel, and eventually collected into "The Oz Omnibus." Young's style is gloriously cartoony. I've included an image of the Omnibus cover.
Hope that sparks some ideas!