Few sites are devoted to prison labor, but a number of sites address the issue along with other issues.
American Gulag, News and resources on Prisoners, Prisons, and Prison Abolition. Contains a lot of information on prison issues, but the word "anarchist" is used quite frequently, perhaps too much so.
Missouri Prisoners Labor Union. Frequently updated site covering more than just prison labor issues.
Prison Activist Resource Center - Prison Labor. Lengthy list of links to the DOC industry sites of the several states and the feds. Very useful.
Prison Legal News. Monthly newsletter. I've read a few issues. Well done and worth subscribing to, even if only to support the cause.
The Wire, Prison-Related News. Presented from the San Francisco State University servers. I suspect it is student-run - and I mean singular. Lots of links to articles and has been publishing for a couple of years.
The Other Side of the Wall. This site is no longer maintained, but anticipating that, the owners archived their nine years of work very well. Still a good resource.
Torture in Texas Prisons. Rather direct name, and (at times) equally direct content. Use the bar on the left to find prison labor information.
ACLU - Prisoner Rights. What set of links would be complete without the ACLU? On the right side is an entry for "Prison Industrial Complex." The content isn't always on-point.