Sutherland started programming on the TX-0, which was widely accessible on the MIT campus, but Sketchpad was definitely done on the big gun, the TX-2, which was still inside Lincoln Laboratories. (Sutherland's uncle-in-law
https://en.wikipedia.org/wiki/Ivan_Getting helped get him into Lincoln Labs. See
https://www.computerhistory.org/collections/catalog/10273819... .) There's an active TX-2 emulation project at the moment
https://tx-2.github.io/ , which has the primary goal of getting Sketchpad running.