I don't think there is a systematic way to search for handwritten manuscripts, and they are rarely available on the web. One collection is here: http://www.claymath.org/node/1370 If you press on the pop-up menu "Publications" in the top, you find manuscripts of Klein, Quillen, Riemann and others. University of Uppsala has Beurling's archive: http://www.math....


Dirac Shoebox Collection at Florida State University contains various handwritted notes by Paul Dirac. A part of it is available online: https://fsu.digital.flvc.org/islandora/object/fsu%3Amarksstaging

Only top voted, non community-wiki answers of a minimum length are eligible