Looking at the ‘using the terrain tool’ documentation, it says to load an example tileset. Where could I view a list of all available tilesets, or download more?
The examples are usually shipped with Tiled (included in the .dmg for OS X or installed to the installation folder on Windows). You can also find them in the source repository in the examples folder:
For finding any other freely available tilesets I refer to OpenGameArt.org.