Oracle desupported 9iAS R1 and R2, and pulled the downloads from their web site about 2 years ago. Unless you find someone with the CD you are out of luck.
If you want 9iAS because you have a 9i database, 10gAS might work for you. I think it supports database 9206 and higher.
If you (your company) has purchased 9i AS and lost the installation CD or downloaded installation files, you might try to contact your Oracle Customer Support, explain what happened and I believe they will be able to give you a new copy of required software.
But, if you'd like to simply download it for your own pleasure, I don't see a way to do that.