Yes, the packages are unavailable on nuget.org but fortunately we have been setting up a custom NuGet server (GleamTech NuGet). For example, DocumentUltimate is now here:
Go to Tools -> NuGet Package Manager -> Package Manager Console and run this command:
Install-Package DocumentUltimate -Source https://get.gleamtech.com/nuget/default/
If you prefer using the user interface when working with NuGet, you can also install the package this way:
GleamTech has its own NuGet feed so first you need to add this feed to be able to find GleamTech's packages. Go to Tools -> NuGet Package Manager -> Package Manager Settings and then click the + button to add a new package source. Enter GleamTech in Name field and https://get.gleamtech.com/nuget/default/ in Source field and click OK.
Go to Tools -> NuGet Package Manager -> Manage NuGet Packages for Solution, select GleamTech or All in the Package source dropdown on the top right. Now enter DocumentUltimate in the search field, and click Install button on the found package.