AS/400 might be costly. I somehow think you'll have a lot more luck finding a J2EE (that's a Java engine, no?) developer. For a short-term contract you're probably looking at somewhere around 8,000-10,000 yen per hour. That's a hunch based on my experience. My very first job with zero experience in anything besides playing around w/ WinNT at home was about 5,000-6,000/hr (to the company of course). I was kind of pimped out b/c I could speak both langauges. Maybe not an issue for you though.
Panache (you can google it) is a head shop that has a lot of people. Since your wants are specific they may not do their standard thing of throwing you a bunch of underqualified applications.