Installing just Pages 09 from old iWork 09 Disk on Yosemite
The simplest solution would be to install it all, and then delete Numbers and Keynote from the iWork 09 folder.

For a brief time, I believe Pages 09 was offered in the Mac app store as a stand-alone program.  If you have that version on an old backup, that would be one other option.  The disc installer cannot be easily modified; and, it puts several files in /Library/Application Support/ and maybe other locations so copying just the Pages.app from your other computer is not a viable option.