This request makes sense for a lot of large enterprises where internet access may be restricted for security considerations.
Steps to accomplish this:
The overall approach is to download the required files on a computer that does have internet access. Then later use those bits on an internal network to install on multiple computers that do not have internet access.
1. Download SSDT standalone installer for VS 2017 (pick a language of your choice from the list)
2. Download Visual Studio Shell (vs_sql.exe) also known as VS SQL SKU