“Captured Japanese maps” is a term only in use in American English and would not be useful for research in other languages. The key term “gaihōzu” 外邦図 (used confusingly for current Japanese home islands as well) and “gunji chizu” 軍事地図 (military maps) may be used to launch your research in other databases. In addition, try the keyword search using agency names, such as "Sanbo Honbu" (参謀本部)、"Rikuchi Sokuryobu" (陸地測量部) or "Suirobu" (水路部). Note that if you are searching the title of a map sheet or series, you may need to use the older form of the kanji 図, that is: 圖.
The following major sites have either digitized and/or cataloged their collection of gaihōzu. Foremost among these is Japan’s National Diet Library. While not scanned, every sheet of some 15,000 has been individually cataloged. Their gaihōzu reference guide is extensive; it includes Japanese nautical charts and useful index maps: 外邦図（一覧）.
The Tohoku University's Gaihozu Digital Archive makes accessible approximately 12,000 maps (72,000 sheets) held in their collection through interactive index maps. The pioneer in digitized gaihōzu maps, the website is in both English and Japanese.
Ochanomizu University Library is home to a collection of some 13,000 sheets, of which they have scanned 73 of their gaihōzu military maps. Called Heiyō chishizu, these maps cover the southwest Pacific, each sheet replete with annotated information. A summary of their collection, with unique holdings, is explained in "Introduction and Overview of the Gaihozu Collection in Ochanomizu University." The portal to their digitized collection can be entered here at 外邦図コレクション (in Japanese, the last listing in the middle of the page will take you to the Heiyō chishizu 兵要地誌図 spreadsheet with links to the scanned maps). Digitized maps cannot be downloaded at this time.
Gaihōzu Kenkyūkai: A study group founded in 2002 for the exchange of information and reporting on research on gaihōzu. Their 外邦図研究ニューズレター(Gaihōzu Kenkyū Nyūzuretā) is published online every year or two and provides a gold mine of recent scholarship and digitizing efforts, as well as interviews with individuals involved in the map-making process during the Pacific War. Articles are in Japanese, including topics concerning maps of the Japanese homeland (and therefore not technically gaihōzu) along with occasional abstracts in English. Links provided on their homepage are to digitized collections, articles in English and Japanese, books (in Japanese), map indexes, and other resources published previously from within their newsletters.
In the No. 11 newsletter from the study group above: ハワイ大学マノア校ハミルトン図書館における外邦図、
Gifu Prefectural Library has built up an extensive collection of gaihōzu from donations and reproductions from other institutions, particularly Tohoku University. While not digitized, their holdings are accessible through metadata, including geographic coordinates of individual sheets. Access to metadata via their spreadsheet (click on tabs on the bottom) is also possible. In Japanese.
Academia Sinica's Taiwan e-Learning and Digital Archives Program (TELDAP) has Japanese colonial-era maps of China and Taiwan. The search platform is in English and scanned maps are viewable as static images (but not zoomable) with the search term "Japanese maps." The Academia Sinica Digital Maps Archive Union Catalog 中央研究院地圖數位典藏整合查詢系統 (Chung Yang Yen Chiu Yuan Ti T'u Shu Wei Tien Tsang Cheng Ho Ch'a Hsün Hsi T'ung) holdings of scanned Japanese imperial maps are searchable through a Chinese language interface here. Unfortunately, at this writing the images can be viewed to only a limited enlargement.
Links to major articles written in English by the leading Japanese scholar of imperial Japanese maps are the following:
For primary resources about Japanese imperial maps, declassified documents are increasingly made available at the Japan Center for Asian Historical Records (National Archives of Japan). You can initiate a search in English but examining the results will entail advanced facility in Japanese.
The pioneer in cataloging Captured Japanese Maps is University of California, Berkeley. While not scanned at this writing, the library's bibliographic records for many of the map sets have served as a model for other institutions. Keyword search by the Romanized Japanese series title in their UCB Library Catalog.
Library of Congress may have one of the most extensive collection of Japanese imperial maps and charts in the world. It is not clear whether all their holdings have been cataloged. Japanese scholars have visited the library collection to photograph maps and charts to facilitate access back at their home institution in Japan. A few sheets of thousands have been scanned; a possible place to begin online viewing may be here. Cataloged records may be retrieved through an advanced search in their online catalog. The Geography and Map Division is creating a list of their holdings and interested researchers may contact Setsuko Means at smea [at] loc.gov.
Stanford University Libraries is completing scanning of some 120 Gaihōzu map sets, making them available through their interactive index maps. There are different options for downloading.
Clark University has a finding aid to their holdings of some 10,000 sheets which can be found in their library catalog here.
The University of Connecticut has scanned their collection of Japanese military maps of Sakhalin here http://archives.lib.uconn.edu/islandora/object/20002%3ASakhalin?display=grid
There are many other American academic libraries holding Captured Japanese Maps--beneficiaries of an appreciative Army Map Service following World War II which established a program for distributing surplus military maps. (Hence, the AMS serial number stamped on sheets in the lower right corner.) From the initial 45 member depository libraries to 150, only 43 received captured Japanese and German maps according to a former Defense Mapping Agency staff member. (Nicoletti, Frank T. "US Army Topographic Command College Depository Program." Special Libraries Association Geography and Map Division Bulletin 86 (1971): 2-3.) Alternatively, details may be available from the National Archives which has a record box of the captured map distribution: NARA RG 226, Stack 190 5/30/7, Box #229.
The National Library of Australia has digitized their collection of most if not all of their Japanese imperial maps. The scanned maps are viewable in their TROVE digital repository. Search in Japanese or by the Romanized Japanese series title. Cataloged-only maps may be searched in the NLA online catalog.
For primary resources about Japanese imperial maps in our library here at Hamilton, there is the four-volume Gaihō sokuryō enkakushi (外邦測量沿革史).