Creating custom brushes in Photoshop is a great way to add texture, or repeating images to work. There are lots of online resources for custom photoshop brushes, such as on Deviant Art, some are available under a creative commons license. The easiest way to get what you want done, is to do it yourself!

Creating custom brushes is pretty simple, and most of the time are just made of simple shapes or textures that in combination with brush options are used to create creative/interesting results.

Creating the Basic Shape

I created a half-moon shape using the pen tool, and then flattened it. Right-clicking on the layer selects the object.

Once the object outline is selected (or the area around it, if preferred) Go to EDIT => DEFINE BRUSH PRESET.

Name the brush accordingly, and your brush is done!

Selecting and Modifying Your New Brush!

To select your newly made brush, select the brush tool (B), and under the Options panel, either select it under the Brush select box, or right-click on your canvas to access the brush selection box.

Also F5 brings up your brush panel which gives you access to many brush options, allowing you to make changes to the frequency, and the appearance of the brush strokes.