Government reports and databases are an excellent source of statistics to support an argument. Criminal justice statistics, including drug usage, can be found in multiple publications that are released regularly.
Sourcebook of Criminal Justice Statistics - This title is now available as an online publication with an archive covering the years 1994 - present. Older editions are available in the US Doc collection at US Doc J 29.9/6: year.
Uniform Crime Reports series - The online archive of this title covers 1995 - 2011.