Sometimes, libraries or non-profit archives like the Internet Archive (archive.org) have scanned copies of older editions available for borrowing. You can search there, though availability is not guaranteed.
ICSE Class 9 Mathematics , textbooks are primarily available through publishers and educational platforms. The official icse math class 9 book pdf
Several trusted educational platforms provide chapter-wise PDF downloads, solutions, and study notes for Class 9 ICSE Maths: icse math class 9 book pdf