You could create your own. It only needs an image with tiles of size 32 x 32. Since I'm feeling generous, I'm going to provide you with a simple image that should work with that code.
You will have to put save it to your computer, put it in the correct folder/directory, and rename the image or change the filename ("tileset") in this line of the code:
if (!map.load("tileset.png", sf::Vector2u(32, 32), level, 16, 8))
Please note that I am no artist.