If you've already had your attunements you may want to contact the Center for Reiki for governs the Reiki Certification.
Not sure if you are following & using William Rand's books, etc. but whether you are using his or whomever company the place that you received your classes & attunements from was certified through should be able to certify & attune you,
I understand that you have already received the Master Attunement & have been actively working with Reiki Energy & Healing, but many places will still require you to have a follow up/refresher attunement through them in order to certify you.
It's basically their seal of approval before giving you the formal signed paper.
Think of it as a Baptism or a Marriage, etc. you go through the Committal Ritual Service not just sign & stamp a piece of paper.
This is just from my experience, you may be able to find someone who will write you up a Certificate without requiring an Attunement.
My advice would be to check your Manuals & contact the company listed on them for assistance.
Best Wishes <3