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You will likely receive more responses if you include the AS/400 topic as a flag. Anyway....
Some if depends on your experience and career desires. The COMMON organization (https://www.common.org/certification/) offers certifications. IBM offers certifications (https://www.ibm.com/certify/). I believe both have practice exams available. There is a discount for COMMON exams if you are a member. I know at the COMMON conferences (spring and fall) the IBM exams are offered at a discount.
There are other organizations that offer exams, but most of them don't carry the weight that the above certifications do.
A little more info on your experience and goals might help us guide you better.
As you are working on this technology, this must be helpful for understanding this certification. Please check here, https://www.ibm.com/certify/