If you already have a licensed SQL Server 2012 RTM media, you should apply immediately. SP4 is the final cumulative update (build 11.0.7001.0). Microsoft still hosts service packs for historical reference:
https://your-vlsc-portal/download/sqlserver2012standard_x64.iso download sql server 2012 standard new
You cannot buy a "new" license for SQL Server 2012 today. However, you can use : If you already have a licensed SQL Server