GTO弁護士の件で盛り上がっているのでざっとインターネットからかき集めて見ました。基本的には80%なのですが、たまに70%や90%を含む場合もあり。originatedなのかproducedなのかdistributedもメチャクチャ。
下記には幾つかの記事、論文がありますがすべて1998年12月4日と12月5日に東京で行われたユニセフグローバルフォーラムでのRalf Mutschkeの発言がもとになっているようです。朝日新聞の同フォーラムに関する記事では「商業児童ポルノの80%を製造または頒布」となっているためこれが一番正しそうです。
また、1999年の児童ポルノ禁止法成立時のヒューストン・クロニクルの記事ではGTO弁護士が「日本産のポルノは全世界の80%という統計もあれば、全世界の20%という統計もある。私たちは本当のところ、よい計り方がわからない。けれども、日本がインターネットにおける児童ポルノのNo.1プロバイダーであることは確かだ」と語っています。
また、例のGTO弁護士の記事ではこう発言しています。「当時、日本は規制する法律もなく、日本から発信される画像がネット上に蔓延（まんえん）していた」このことからGTO弁護士の認識として、昔も今も1998年当時としては頒布に重点を置いていることが分かります。
http://www.aifs.gov.au/nch/pubs/issues/issues15/issues15.html
For example, Interpol reports that most of the child pornography available on Internet sites now originates in Japan due to their lack of laws prohibiting its production (Editor 1999)
http://www.nordicom.gu.se/cl/publ/archive/NL_1_1999.pdf
P13 According to estimates from Interpol, most of the child pornography available on Internet sites world-wide originates in Japan.
http://www.popcenter.org/problems/child_pornography/PDFs/Graham_2000.pdf
Noting the extent of the problem, Interpol, the international law enforcement organization, estimated last year that eighty percent of the child pornography available over the Internet was produced in or distributed through Japan.
See Interpol ties 80% of world's child pornography to Japan, ASAHI SHIMBUN/ASAHI EVENING NEWS, Dec. 4,1998, available at 1998 WL 22369868 [hereinafter Interpol ties] ("Germany . . . [has] amended laws to enable police to charge Internet service providers that allow child pornography to be distributed electronically").
http://www.time.com/time/world/article/0,8599,2054271,00.html
According to estimates from Interpol, as much as 80% of the child porn available on commercial sites worldwide originates in Japan. A police study found more than 3,000 Websites based in Japan distributing pornography, 40% of them featuring children.
http://www.csmonitor.com/1998/1216/121698.intl.intl.1.html
Interpol, the international police organization that hosted the Lyon meeting, estimates that 80 percent to 90 percent of the world's commercially distributed child pornography is produced in Japan, although mostly foreign children are depicted.
http://www.chron.com/CDA/archives/archive.mpl/1999_3139725/japan-oks-ban-on-child-prostitution-child-porn.html
Interpol, the international police agency, has estimated that between 70 percent and 80 percent of the child pornography available on the Internet came from Japanese sources.
"Some figures say 80 percent and some say 20 percent; we really don't know a good way of measuring it," said Keiji Goto, an official at the National Police Agency who campaigned on behalf of more stringent laws. "But it is sure that Japan is the No. 1 provider of child pornography on the Internet."
"Interpol, the international police organization, estimates that 80 percent of the world's commercial child pornography materials found on the Internet are suspected to be produced or distributed from Japan."
ECPAT estimates that 60 to 80 percent of all pornographic images of children are distributed from Japan.
