Supplement to Iman Mahmoudinasab's answer
For SQL Server 2016, this is where to find the files:
https://www.microsoft.com/en-us/download/details.aspx?id=52676
Note that the files are in the list but you may need to scroll down to see/select it.
From SQL Server 2017 onwards, things change:
"Beginning with SQL Server 2017 SMO is distributed as the Microsoft.SqlServer.SqlManagementObjects NuGet package to allow users to develop applications with SMO."