I spent yesterday hearing all about the archives relating to Eric Gill and his Associates at Notre Dame University from its curator John Sherman. Look at http://www.library.nd.edu/rarebooks/collections/rarebooks/gill.shtml at the Hesburgh Library, University of Notre Dame, Notre Dame, IN 46556. What a fantastic collection!
I've see the one in Ditchling Museum and the British Museum. And heard about the William Andrews Clark Library in the University of California Los Angeles., the West Sussex Records Office and the V&A print collection.