Additional part libraries are available through the QCAD menu:
Help > Browse for Add-ons
You can use any DXF (or if you are using QCAD Pro DXF and DWG) files in the part library. Simply save your DXF / DWG files into a dedicated folder on your disk and configure QCAD to use that folder as part of the part libraries:
Edit > Application Preferences > Widgets > Library Browser > Library Sources
Depending on what you are looking for, the Internet might be a good source of free and payed part library items. For example, google for "free DXF tree symbols" or "dwg nuts and bolts", etc.