This site maintains a version history for VEGAS Pro where older files are stored for users with compatibility issues on newer hardware.
Some users on Reddit recommend searching the Internet Archive for snapshots of the original Sony Creative Software site or mirrors of the install files. Sony Vegas 9.0 Pro Download
Downloading today is complex because the software is over 15 years old and the brand has changed ownership. Sony sold its creative software line to MAGIX in 2016. 1. Official and Legacy Downloads This site maintains a version history for VEGAS
There are listings for Vegas Pro 9.0 beta or trials, though these may be incomplete or limited in functionality. 3. Risks and Important Safety Warnings Sony sold its creative software line to MAGIX in 2016
If you do not have a pre-existing license, finding a legitimate download for version 9.0 specifically is difficult.