Microsoft ended mainstream support for SQL Server 2008 on July 8, 2014, and extended support on July 9, 2019. This means that you won't receive any security updates or support from Microsoft.
You're looking to download the full version of Microsoft SQL Server 2008. Here's what you need to know: ms sql server 2008 download full version
If possible, consider upgrading to a newer version of SQL Server, such as SQL Server 2019 or Azure SQL Database, to take advantage of improved features, performance, and security. Microsoft ended mainstream support for SQL Server 2008