Umbraco.Elasticsearch is only available as a NuGet package and can be installed into an existing solution using:

Install-Package Umbraco.Elasticsearch

This will install both the UI package (Umbraco.Elasticsearch) and the core library (Umbraco.Elasticsearch.Core).

Umbraco.Elasticsearch takes a dependencies on the Full UmbracoCMS package, while Umbraco.Elasticsearch.Core only depends on the UmbracoCMS.Core package. This means if you have a solution with multiple projects you can take a dependency on the Umbraco.Elasticsearch.Core package in all your non-web projects and avoid pulling in unnecessary plugin files.

Once installed you will have to configure the integration

Umbraco.Elasticsearch is not available as an Umbraco package.