One of the great new features in Inkscape, is the possibility to use libraries. There are a few nice libraries already available on installation, but obviously what you would want, is to add your own libraries.
So here's the folder where you can add your own libraries:
- Linux: ~/.config/Inkscape/symbols/
- Windows: C:\Program Files\Inkscape\share\symbols
- Mac OS: (don't have a Mac... you tell me)