The most practical thing I can suggest is to find someone with a Mac running between El Capitan (10.11) to High Sierra (10.13) with faster Internet access and have them select to download Mojave from the App Store.
It will appear in their Applications folder. If they copy the "Install macOS Mojave" file from that Applications folder to a Mac-formatted USB thumb drive I believe you can use that without any problem. I don't think it will matter for that file that you would not be having their Apple ID/password to install it. Perhaps someone else can correct me if I am wrong.
If that installer doesn't work for you though, then the only guaranteed way would be to have them download that file and then turn it into an actual USB-bootable install media by following the directions for "Mojave" contained in this help article:
How to create a bootable installer for macOS - Apple Support
This would require you provide them a 16gb+ USB thumb drive formatted as HFS+. And to save you and them a headache you would want to rename the USB drive before you give it to them to be "MyVolume" so that they could cut and past the needed command line command directly from the help article.