Your phrase "source for this claim" implies to me that you didn't read the Rolling Stone article under discussion. It is a "source for this claim."
Quoting from the article: "But don't worry your pretty little heads over the epidemic of far-right insurrectionism that followed the election of Barack Obama: all told, according to a forthcoming data analysis by Neiwert, there have been 55 cases of right-wing extremists being arrested for plotting or committing alleged terrorists acts compared to 26 by Islamic militants during the same period. The right-wing plots include the bombing of a 2011 Martin Luther King Day parade in Spokane and the assassination of abortion doctor George Tiller in 2009."
It links to http://www.csgv.org/issues-and-campaigns/guns-democracy-and-... which includes a list of "incidents of insurrectionist violence". It also links to a book, "Over the Cliff: How Obama's Election Drove the American Right Insane" by the aforementioned Neiwert.
Quoting from the article: "But don't worry your pretty little heads over the epidemic of far-right insurrectionism that followed the election of Barack Obama: all told, according to a forthcoming data analysis by Neiwert, there have been 55 cases of right-wing extremists being arrested for plotting or committing alleged terrorists acts compared to 26 by Islamic militants during the same period. The right-wing plots include the bombing of a 2011 Martin Luther King Day parade in Spokane and the assassination of abortion doctor George Tiller in 2009."
It links to http://www.csgv.org/issues-and-campaigns/guns-democracy-and-... which includes a list of "incidents of insurrectionist violence". It also links to a book, "Over the Cliff: How Obama's Election Drove the American Right Insane" by the aforementioned Neiwert.