As has been highlighted, using the media that the product came with is only the beginning to ensuring that your Windows SBS 2011 Standard system is kept up-to-date. You'll not only find critical updates and service packs here, but you'll also find a number of important add-ons and utilities for all versions of SBS that will really help you over the life of the product:
Migrate to Windows Small Business Server 2011 Standard from Windows Small Business Server 2003: http://www.microsoft.com/download/en/details.aspx?id=14570
Migrate to Windows Small Business Server 2011 Standard from Windows Small Business Server 2008: http://www.microsoft.com/download/en/details.aspx?id=19219
Migrate to Windows Small Business Server 2011 Standard to New Hardware: http://www.microsoft.com/download/en/details.aspx?id=25237
Windows Small Business Server 2011 Standard Installation Guide: http://www.microsoft.com/download/en/details.aspx?id=13913