You can source official and community-driven PKT files from these primary locations: Official Autodesk Sample Library
Q: How do I install Civil 3D subassembly PKT files? A: To install a Civil 3D subassembly PKT file, extract the file, open Civil 3D, navigate to the Subassembly Composer, and click on the "Install" tab. civil 3d subassembly pkt download updated