It is a Mac font (TrueType though) and I don't think it is available for purchase.
At the link below, you can look at some alternatives.
http://www.identifont.com/list?3+Chalkduster+0.30+27VX+1+4AHB+2+3OB5+2+6...

The font Eraser available on DaFont is similar as well.