Here's a quick research note: a list of Wilfrid Voynich's archives...
- The Beinecke Rare Book & Manuscript Library not only has the Voynich Manuscript itself ("Beinecke MS 408"), but also various other WMV- and VMs-related materials in accompanying boxes (marked A to N). This includes various photographs (in box D), which are also accessible on the Beinecke main site.
- The Grolier Club in New York has 8 boxes of WMV's personal and book-trade effects, 1916-1948 (this also contains correspondence carried on by Anne Nill after WMV's death in 1930, as well as some writing by Ethel Voynich).
- The London School of Economics has 15 boxes of WMV's "Pamphlets and treatises, concerning the Polish struggle for independence, 1846 - 1914", though these have now been withdrawn and are accessible only on microfilm. Ref: COLL MISC 0261.
- The Wellcome Institute Library has a box of correspondence with WMV (dated 1922-1924), (though I don't know how many letters this contains, probably not many). Ref: WA/HMM/CO/Chr/A.110 -- Box 89.
There may well be more, but that should be enough to keep any researcher going for a bit... :-)