This whole set of square brushes comes with Photoshop and they just need to be loaded into your brush palette to enable you to start using them.
To load the square brushes into your brush palette:
With a document open and the brush tool selected, right click the canvas.
Click on the arrow which is top right on the brush selector.
Choose Square brushes from the list.
You should now be able to access all the Photoshop square brushes 🙂