The UvA holds a large collection of letters. They are listed in the UvA catalogue. Select Advanced search and then select Manuscripts in the formats filter. For the search windows select an option from the drop-down menu:
Use the filter period to find letters from a specific period. If you are looking for a letter with a specific date, select Browse instead of Search, and then select Date as search key. If, for example, you are searching for a letter sent on 21 June 1945, enter 1945-06-21.
Since not yet all letters are listed in the online catalogue, you are advised to search the card catalogue in the Reading Room of the Special Collections, as well.
The Catalogus Epistolarum Neerlandicarum lists letters of the collections of a number of Dutch heritage institutions, among which are the UvA, the Koninklijke Bibliotheek (National Library of the Netherlands) and the Letterkundig Museum. Select a search key from the drop-down menu.
Published correspondences can be found in the UvA catalogue by combining the name of the author and the search term brie? or corresp?.