An Ethernet controller is a physical device used by a computer in order to communicate with other devices on as network. It is not something which can be downloaded.
It is utterly inane to ask how to download something when offfline. The very definition of "download" is to transfers something via an "online" connection.
The *SOFTWARE* drivers for any particular Ethernet adapter can usually be downloaded from the website of the mfg. If your compyuter at home or wherever cannot access the internet to download something then you will have to access a computer which is "online" such as at the home of a friend, a library, internet cafe, or hell, some passing stranger at a location providing free access such as a restaurant.