Do not download v5.6. The "BEST" firmware for the FortiGate 100D is FortiOS 5.4.13 (build 1180).
run FortiOS 7.x or higher; these versions are reserved for newer models like the 100F or 120G. How to Download Fortigate 100d Firmware Download BEST
If you want, I can:
While 6.2 is the latest it can run, it is largely considered a "legacy" train. For modern production environments, replacing the hardware with a G-series or F-series model is recommended. How to Download the Firmware Do not download v5
— there is no "best" unofficial source. Only official Fortinet downloads are legitimate and safe. Fortigate 100d Firmware Download BEST