In the default sets, the chalk brushes are a good start. Add in some angle, size and opacity jitter to get away from the hard edges and you've got a good starting point.

If you're up for delving deeper into the brushes, open up the brush selector and hit the little arrow on the top right (above the Create New Preset button) and that should give you a menu with a whole load of new brushes. One of those sets is called 'M Brushes' and has a whole collection of weird and wonderful brushes. I like 'sumi 2' from that set or 'rough wash'.

Equally, it's always worth delving through deviantArt for 'concept texture brush'. Here's a great set that turned up:
http://browse.deviantart.com/?qh=&se...brush#/d298fus