You can download the ISO file without a product key from the Microsoft Evaluation Center website.
You can often find older developer versions by joining the free Visual Studio Dev Essentials program. Once logged in, search the "Downloads" section for SQL Server 2012 Developer. Sql Server 2012 Developer Edition Download Iso
Because SQL Server 2012 is a deprecated version, the availability of its media is restricted: Visual Studio Subscriptions : If you have a paid Visual Studio Professional or Enterprise You can download the ISO file without a