You should take a look at the US Census Bureau's Statistical Abstracts from 1950-1994 which can be accessed here: www.census.gov/prod/www/abs/statab1951-1994.htm. Open the census report for each year you are interested in, and in the table of contents you will find a section titled 'Law Enforcement, Federal Courts and Prison'. This section provides charts that contain different types of information regarding US inmates. If you are looking for information from a specific year and you can't find it in the census from that year, try looking at the census from the following year. I hope this is helpful!
print copies
In addition to the online copies of Statistical Abstracts, you might be able to find them in print copies at a Federal Depository Library. Also, it is important to know that depository collections are accessible to the public, even those contained in an otherwise private (typically academic) collection.