You can download the macOS 8 ISO from various online sources. Make sure to use a trusted site to avoid malware.
For emulate macOS 8 on Basilisk II, you'll need the ROM, I reccomend to use the Old WORLD ROM, you can find it on:
To install macOS 8 on Basilisk II, follow these steps:
If you are on macOS you can use the official GUI and the configuration is similar to SheepShaver
For Windows and Linux users, you will need to edit the configuration files manually.
Basilisk II Setup Guide