A quick Google search for “Mac OS ISO file download 64-bit for VirtualBox” will yield dozens of sites claiming to host ready-made ISOs. Examples include:
No. The installer will crash or run unbearably slow. Minimum 8 GB host RAM, allocating 6 GB to the VM. Mac Os Iso File Download 64-bit For Virtualbox
This is the gold standard. Apple freely provides macOS installers via the App Store. You can convert that installer into a bootable ISO usable in VirtualBox. You will need access to a real Mac or a working macOS VM to download the installer. A quick Google search for “Mac OS ISO