Moore vs. Texas - There are many references to this case, as well as the actual text of Supreme Court case, online. Here is a nice summary of it from the Constitution Accountability Center.
Atkins Case - A fine summary and link to full text of the case from LexisNexis, a database of cases.
Hall vs. Florida - Cornell University's Legal Information Institute provides a bulleted lists of main arguments in this case.
8th Amendment - Summary and history of the 8th Amendment, by the National Constitution Center
Ex Parte Briseno - An article from the American Bar Association titled "Supreme Court Finds Texas Briseno Factors for Establishing Intellectual Disability Unconstitutional." Helps to explain its definition and application.
IQ Tests - The Origins and History of IQ Tests from Interesting Engineering. Also includes a discussion of the relevance and controversies surrounding IQ tests.
Washington Laws regarding Intellectual Functioning and the Law - Try the search terms (notice the "-education" as there are lots of special education references): ~ washington state law "intellectual functioning" -education ~