If you don't find yours listed here check the MARC Code List for Organizations Database (Network Development and MARC Standards Office, Library of Congress) at: http://www.loc.gov/marc/organizations/org-search.php

@mainagencycode: constrained to ISO 15511 (ISIL)

Note that you will need to preface the code with a two - letter country code. For example, Austin Presbyterian Theological Seminary's code is "US-txaup"

Repository Code
 African American Library at the Gregory School  
 Albert and Ethel Herzstein Library, San Jacinto Museum of History  US-txhsjm
 Austin History Center, Austin Public Library  US-txau
 Austin Presbyterian Theological Seminary  US-txaup
 Baylor University, Armstrong Browning Library  US-txwbb
 Baylor University, Keston Center for Religion, Politics, and Society  txwabkc
 Baylor University, The Texas Collection  US-txwabtc
 Baylor University, W.R. Poage Legislative Library  US-txwabll
 Concordia Lutheran College  US-txauc
 Daughters of the Republic of Texas Library at the Alamo  US-txsadr
 East Texas Research Center, Stephen F. Austin State University  US-txnasfae
 H.J. Lutcher Stark Center for Physical Culture and Sports  US-txaulsc
 Haines Medical Library at Scott & White  US-txtemh
 Harry Ransom Humanities Research Center, University of Texas at Austin  US-txauhrh
 Houston Academy of Medicine-Texas Medical Center Library, John P. McGovern Historical Collections and Research Center  US-txhmc
 Houston Public Library, Houston Metropolitan Research Center  US-txh
 Lamar University's Archives and Special Collections  US-txbeal
 Rice University, Fondren Library, Woodson Research Center  US-txhr
 Robert E. Nail, Jr. Historical Archives at Old Jail Art Center  US-txalyoja
 Sam Houston State University, Special Collections  US-txhut
 San Antonio Municipal Archives  
 Southern Methodist University  US-txdadf
 Southwestern University, Special Collections & Archives  US-txgeos
 Special Collections and Archives, Texas A & M University-Corpus Christi  US-txcctam
 Tarlton Law Library, University of Texas at Austin  US-txul
 Texas A & M University Cushing Memorial Library  US-txcm
 Texas A & M University Kingsville, South Texas Archives  US-txkt
 Texas General Land Office Archives and Records  US-txauglo
 Texas Southern University  US-txhtsu
 Texas State Library and Archives Commission  US-tx
 Texas State University San Marcos  US-txsmtsu
 Texas Tech University Southwest Collection/Special Collections Library  US-txltsw
 Texas Woman's University, the Woman's Collection  US-txdw
 Texas/Dallas History and Archives Division, Dallas Public Library  US-txda
 The University of Texas at Austin. Alexander Architectural Archive  US-TxAuUAA
 The University of Texas at Austin. Benson Latin American Collection  US-txula
 The University of Texas at Austin. Dolph Briscoe Center for American History  US-txuth
 Trinity University Special Collections and Archives  US-txsat
 Tyrrell Historical Library  US-txbea
 University Archives and Special Collections The University of Texas at Tyler  US-txtyu
 University of Houston Libraries, Special Collections  US-txhu
 University of North Texas Archives  US-txdtuna
 University of North Texas Music Library Special Collections  US-txdnmu
 University of St. Thomas Archives  US-txhosta
 University of Texas Arlington Library, Special Collections  US-txaruls
 University of Texas El Paso  US-txeu
 University of Texas M.D. Anderson Cancer Center  US-txua
 University of Texas Medical Branch, Moody Medical Library  US-txum
 University of Texas San Antonio  US-txsau
 UT Health Science Center San Antonio  
 UT Human Rights Documentation Initiative  
 Vietnam Center and Archive, Texas Tech University  US-txlbtuv