Only Websphere or Weblogic is supported for JDE. While you might be able to use other J2EE servers, you're venturing into completely unknown territory - "here be dragons".
If you're concerned about pricing - then talk to an experienced partner (like myself) who can explain how JDE Technology Foundation works and provides extremely competitive pricing for your implementation. Believe me, Weblogic is expensive if purchased on the standard per-core model - but JDE has provided a solution for all customers.
However, if you think JDE is "free" or everything supporting JDE should be "free" - then you're going to be going down a path where you'll be on your own, and you'll likely encounter the wrath of the entire software industry - not just Oracle. JDE is not free, its an Enterprise product provided by one of the largest software companies on the planet, and it costs Oracle hundreds of millions of dollars to maintain it. Although Oracle supports Open Source, Open Source ISN'T free. It doesn't mean free. It means that you can download and modify the code - but at the end of the day, someone has to get paid to keep the code current.