It is available from the public OTN download page (which hosts only older versions like 11.2.0.1). Unauthorized third-party sites should never be used due to security and licensing risks.
unzip <installation_file>.zip
(production or testing requiring exact version), you must purchase a valid Oracle license or already have a support identifier.
Before installation, ensure your environment meets these minimum specifications: : Minimum 1GB RAM (2GB+ recommended).