Viz Media, the current English publisher of the Shaman King & Ultimo manga series in English-speaking territories, has silently removed both of these series from purchase from all digital platforms. Unless purchased previously, neither of these series can be purchased in a digital format anywhere in the world. Back in July with Shaman King, and then later in September with Ultimo, Hiroyuki Takei has taken back the distribution rights from the Japanese publisher Shueisha. Particularly for Japan, this take-back was for both the digital volume as well as the print editions. Currently the volumes are still listed for sale for the print/paperback editions only on Viz’s website, and has all references to the digital volume sales removed.
Internationally, the individual publishers responses were mixed. Viz Media held the only remaining digital license for the manga series, so outside of Japan, there is no comparison. For print volumes, publishers are either only continuing with reprints (which don’t require Japanese editorial sign-off), or holding off printing new volumes until the licensing issue in Japan has been resolved.
If you had previously purchased the manga from a digital vendor, you should still be able to re-download the volumes. However ability may vary by service and could change over time.
Viz Media is owned by Shueisha, Shogakukan, ShoPro, and the Hitotsubashi Group in Japan, which makes their licensing agreements a bit more challenging. Other international licensors (including Viz Media Europe), work independently of the Japanese publishers, which can help ease publisher transitions. Unless these series licensing agreements are transitioned to Shogakukan (who is currently publishing Hiroyuki Takei’s Hyper Dash! Yonkuro in Japan), it may be difficult for Viz Media to regain the English publishing rights for the series. If you haven’t purchased the physical (print) volumes of these mangas yet, I would highly recommend you do so as we do not know how long Viz Media will be able to reprint/distribute the Shaman King and Ultimo manga series in English.