Just goggle “AS400 Jobs” and you will get many possibilities
I have used AS400JOBS.COM
i tried as400jobs.com site lot of time, but not access this site….
Check out http://www.juju.com and http://www.indeed.com, both poll and aggregate results from multiple job boards and are kept up to date.
Any IT jobs website should be able to list jobs for this technology. Try http://www.dice.com, http://www.indeed.com or any of the other big job websites.
You can have a read of my post about entry level IT jobs at http://www.completeitprofessional.com/career/how-to-get-an-entry-level-it-job