WorldCat is the OCLC union catalog, containing over 43 million records describing books and other items owned by libraries worldwide. Each record contains library holdings, including archival materials. There is also a freely available version of WorldCat.
Archives contain incredible primary source materials. Because these materials are rare, and sometimes valuable, they are rarely allowed to leave the archive.
If you identify archival materials that are located far away, you may be able to get copies of some materials by paying for it. Do not expect that archives will copy materials for free.