MajestiK

Is there a brush that lets you get the pixelated effect? If so, where can I get it?

Rantin Al

You can use the Pencil Tool for pixel art. Hold the mouse button down on the paintbrush tool to select it from the flyout.

Cheers Al

aironite

Or select a fair size paintbrush or custom brush and set the mode to dissolve and adjusting the flow to your taste.

dv8

ladyfinger007

Or, if you are trying to give an effect similar to the Filter>Pixelate>Mosaic...

The best is to create a copy of the base layer, run the filter, then ALT+Click the layer mask icon (second icon in the layers palette) this way, all the layer is masked, and you can now use a white brush to reveal only the parts you want...