I don't know if anyone has ever mentioned this on the board before, but there is a large collection of scanned images of various letters between George Kerr and others, and Edward Paine and others. There may be other kinds of documents there as well. I've only looked at it briefly.
Here's a link to the collection: goo.gl/4SCre4
It's apparently a part of the National Repository of Culture Heritage of the Ministry of Culture. I stumbled upon it tonight while searching for the M. Ottsen (I think Dr. Marcus Ottsen) who reportedly is the author of a few photographs taken during the 228 Incident. (I wasn't able to find too much information about Dr. Ottsen online, and I doubt that there's any correspondence by him or to him in this collection.)
Edited to add: Some of the indexed documents are missing. I can't guess the percentage. Still, there's a pretty good deal of material there.