: Log in to your Adobe account and check under "Plans and Products" or "Registered Products". Your original 24-digit serial numbers are often stored here.
Finding a reliable is a common challenge for designers maintaining legacy systems. While CS5 was a staple of the design world after its 2010 release, Adobe has officially moved to a subscription-based Creative Cloud (CC) model.
: If you are installing an upgrade version (e.g., from CS4 to CS5), you will need the serial numbers for both the current and the qualifying previous version. The Dangers of Using Public Serial Number Lists