The most reliable source is the stock ROM (firmware) specifically for your exact device model.
Scatter files are device-specific, even if they share the same CPU. You can find them in: Download MT6735 Android scatter txt
If a phone is stuck in a boot loop or dead, the scatter file allows re-writing of the preloader and system files to bring it back to life. Typical Scatter File Structure (eMMC) The most reliable source is the stock ROM