If you have a legitimate need for the specific 2013 version (e.g., compatibility with legacy projects), your best bet is to contact Autodesk Support directly. As noted on their forums: “Autodesk supports products up to three versions past the current release. If the download you need is not located within the Virtual Agent, please contact our Support teams to see if your product is still available”. However, be prepared that “unsupported versions can become unavailable at any time”.
The most responsible and safe path forward is to explore the official, modern, and frequently free options provided by Autodesk, such as the current 30-day trial or an educational license for the latest Civil 3D software. These alternatives provide a secure environment to learn and work, ensuring your data is protected and your designs are based on a supported, professional-grade platform. Autocad Civil 3d 2013 64 Bit- Free Download