There's 10 boxes of materials related to Fraggle Rock, in the Lee Fonds (via Dennis Lee) at University of Toronto Libraries. Good call, Grant. There's also a variety of Dark Crystal/Labyrith stuff, including draft lyrics for "Song of Kira"... we don't have a page by that title, so I'm wondering if that was a draft name?
Interestingly, a Bob Stutt image in the archives at Civilizations cites Wikipedia for biographical information. I'm seeing scripts, a blueprint for the Doozer's cave, Tim Gosley's records (correspondence, contract agreement, greeting cards, press clippings, shooting schedule, The Encyclopedia Fragglia), and an puppeteer audition script called "One of Our Fraggles is Lost". A lot of press clippings via the Ontario Puppetry Association, including a 1964 Toledo Blade interview with Rowlf, via Henson. Pic's down currently...
(Of interest, some of the marionettes that they didn't take from the OPA came to the second element of the Peel Art Gallery, Museum and Archives; I work in the third.)
More modern note, they even have photos from the Sesame Street Halloween Party that they held there in 2011, with the local costumed character licensees.