There are relatively few children available for adoption in Japan. One reason is the relative prosperity of the country. But a very important reason is that there is a huge emphasis on the "blood tie" in Japan. Many people who can't parent put a child into an orphanage, but will not release him/her for adoption because they can't imagine a decent person willing to give his family name, etc. to an unrelated person. And international adoption is particularly incomprehensible.
The non-Japanese people who have had the most success adopting from Japan are those who actually are living in the country for a while and have familiarity with the adoption system. However, it is not totally impossible for someone living overseas to adopt, even without using an American agency.
The U.S. State Department website has a very clear description of the Japanese adoption process, which can assist a person trying to adopt without an agency. Go to
Intercountry Adoption Japan.
One thing you need to remember is that not all children available for adoption in Japan meet the U.S. Immigration and Nationality Act eligibility requirements for an adoption visa. If you adopt a child in Japan (or any other country) who is not eligible for an adoption visa, you would have to live overseas with him/her for at least two years before obtaining a regular visa for him/her.
I wish you good luck.
Sharon