Drag and Drop GeoJSON File onto Map
This sample shows how load add support for dragging and dropping GeoJSON files from your local computer file system on to the map and having them render on the map. Also take a look at the
spatial IO module samples
if you would like to support additional spatial data formats such as KML, KMZ, GPX, GeoRSS, GML, and spatial CSV files. Here are some places where you can find some sample GeoJSON files.
Mapping L.A. Boundaries API
Here are some great places to find GeoJSON files to try out. You can also create your own GeoJSON files by drawing on a map