The MISRA C:2012 guidelines (and other MISRA documents) can be purchased from the MISRA webstore at https://www.misra.org.uk/shop in either hard copy of pdf form.
The mappings between the MISRA C:2004 and MISRA C:2012 guidelines can be downloaded as Addendum 1 in the "Resources" section of the Bulletin Board (registration required). https://www.misra.org.uk/forum/viewtopic.php?t=1563