Where Can I Find The Bootanimation.zip For A ROM On GitHub
The bootanimation.zip will not be present in the Android Source Code or the kernel source code..

It's gets included from the vendor blobs while building the system.img ..

So, you'll have to check the corresponding ROM's vendor repository..

For CyanogenMod, it's here: https://github.com/CyanogenMod/android_vendor_cyanogen/tree/master/prebuilt

For Dirty Unicorns, it's here: https://github.com/DirtyUnicorns/android_vendor_du/tree/m/prebuilt/common/media

For Paranoid Android, it's here: https://github.com/AOSPA/android_vendor_pa/tree/nougat/prebuilt/bootanimation

For Pac ROM: https://github.com/PAC-ROM/android_vendor_pac/tree/pac-6.0/prebuilt/common/media

(These links are subject to change, please edit if there's any change or to add any other ROM's link)