It was merged with another fileset. It's here:
Download that and unzip it.
Inside that should be a few folders. One is named "10_3_r183_90".
Inside that folder is a flashplayer_10_3r183_90_mac_intel.dmg
Double click that and it will mount on your desktop, and open a window with the installer.
Right click the installer and choose "Show Package Contents"
Go to: Contents/Resources and double click the Adobe Flash Player.pkg file
The installer will open, ask for your Admin password, and it will install.
Is your processor one of the 32 bit Core 2's?
Is that why it can't be upgraded?