Visual Studio 2017 only has offline Nuget Packages for .NET Core 2.x. How to get online packages?

I think the answer is:

1. Right click on the project and choose Manage NuGet packages.

2. Click on the settings cog in the top right corner of the Nuget packages tab.

3. In the settings dialog (as shown in the screenshot in the question), click on the + button.
4. Give the new source a name and a source of https://api.nuget.org/v3/index.json.

This appears to work for me