I need to find statistics on the incarceration rate of African Americans for the 1930s. Where can I find these statistics?
You are talking about nearly fifty state systems plus the federal system. There is going to be substantial research behind the answer if the work has not already been done for you.
The obvious place to look (after Google) is legal scholarship, as by a lexis or Westlaw search of law journals. Failing that or if you do not have access to one of those databases, your best bet is to ask a law school librarian or legal scholar in either early twentieth-century incarceration or critical race theory for the resources most likely to have the information you need.