You can find the Cubase 5 ISO image on the Steinberg unsupported software page.

Because Cubase 5 is an older version, Steinberg no longer sells it directly. However, they provide official download links for existing license holders through their archive.

Cubase 5 requires a valid USB-eLicenser (dongle) and a registered serial number to run. If you have lost your installer but still have your dongle, these official links are your safest bet. 2. System Requirements & Compatibility