My buddy Kevin Greene just released Visio stencils for SCVMM 2012 R2 that he created. You can find them here.
Here are some free SCCM 2007 Visio Stencils published by Jawad Qazi on the Technet Gallery.
Larry Rayl from Catapult Systems updated the Visio Stencils so they can reflect to SCOM 2012 (RMSe, Resource Pool and etc.). Download link and more about the Stencils you can find here. Happy schema creating .