DoSDude's mojave patcher includes a menu item to download the installer directly from Apple. DosDude is a good source for the Mojave installer if you are unable to get it from Apple.
It also has another firmware update, but it is able to do it without a vid card boot screen. So after you upgrade to High Sierra, then you must swap the new Metal card in and run the Mojave installer. This is important because the High Sierra installer includes a firmware update that is needed also for Mojave. You will be stopped by the installer and told to upgrade to High Sierra first.
