I found it only on the main sites such as Amazon or the dutch version of Bol.com
Here's the amazon link with a current sale on it:
https://www.amazon.com/Kingdom-Hearts-Complete-Novel-Collectors/dp/1975333322/ref=mp_s_a_1_1?crid=W2Y1ZXGAQ9F9&keywords=kingdom+hearts+novel&qid=1655661320&sprefix=kingdom+hearts+novel%2Caps%2C242&sr=8-1
https://www.amazon.com/Kingdom-Hearts-Complete-Novel-Collectors/dp/1975333322
https://www.amazon.com/-/de/dp/1975333322/ref=sr\_1\_1?\_\_mk\_de\_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&crid=1AO54J35B6WAY&keywords=kingdom+hearts+novel&qid=1658389690&sprefix=kingdom+hearts+nove%2Caps%2C178&sr=8-1
Ok fount it on german amazon https://www.amazon.de/-/en/Tomoco-Kanemaki/dp/1975333322/ref=sr_1_3?crid=3FTQB0OCEA7Y1&keywords=k%C3%B6nigreich+herzen+buch&qid=1654422764&sprefix=%2Caps%2C93&sr=8-3