Many times searching for statistics is like following a trail. Sometimes you'll be really fortunate and find a perfect match for your topic, but others you'll need to look closely at the source of the related statistics and dig a little further to see if additional statistics are available from that same entity. For example, you might read an article in a newspaper that mentions some statistics...not quite on your topic, but related, so you'll want to take note of who had collected the statistics and go to their site and look for more information.
For example, let's say you were researching about juvenile delinquency and were reading an April 7, 2014, New York Times article called "When Youth Violence Spurred 'Superpredator' Fear." The article references statistics kept by the Justice Department's Office of Juvenile Justice and Delinquency Prevention. This should prompt you to head to the Justice Department Office of Juvenile Justice and Delinquency Prevention website. There you'll see an OJJDP Research & Statistics section where you can delve in to see the original statistics for yourself. It's a process...
These are some suggestions to begin exploring for statistics related to criminal justice:
