Could you be more specific? Criticizing, how? Certainly there were and are critics of Dr. King's non-violent approach (Malcolm X and the Black Panthers, for example), but they all fundamentally agreed on the same thing: African Americans were severely lacking civil rights (among other rights). Now, if you're asking if there are any sources criticizing his support for civil rights for blacks, you're probably not going to find any reputable sources. All respectable historians will agree that racism is bad. Certainly you'll find white supremacists who will criticize Dr. King, but that's not "good history".