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:
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:
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