If you just need sas2flash or sas3flash, they are included with TrueNAS these days (at least as of 25.4).
/usr/local/sbin/sas2flash
/usr/local/sbin/sas3flash
You will still need the firmware (and maybe the BIOS) image file(s).
But booting from a FreeDOS USB may be safer as it guarantees no I/O through the HBA during the flashing.