Use of Miraculin is approved in Japan as a food additive but not in the US. You may wish to search for a source in Japan. I imagine it should not be too difficult.
I am from the Philippines and I am also researching about miraculin. Miraculously, there were lots of miracle berry trees available in our locality of Davao City, but it is not widely known. The problem is the extraction
I have heard some news regarding its availability. I've heard that there are some companies there in India that processes the berries for its appetizing qualities. You may want to look for some connections there