Ger ready, 'cause here it comes.
First of all, remember that you'll need three manuals: one for the mainframe and one each for the vertical and the horizontal plug-in.
Web sites dealing with manuals, either specializing in test equipment or having a lot of test equipment:
http://www.agtannenbaum.com/
http://www.manualman.com/
http://www.manualsplus.com/
http://www.w7fg.com/
Sites having links to manual resources include:
http://gama.inesc.pt/public/info/repair/f_manuals.html
http://www.repair.faq.org/repair/f_manuals.html
Off the Web, this guy specializes in Tektronix manuals:
Ed Matsuda
POB 390613
San Diego CA 92149
619-479-0225 (voice)
619-479-1670 (fax)
Here's the best Tektronix site in the world for anything having to do with out-of-production Tek equpment:
http://www.reprise.com/host/tektronix/home/default.asp
And they have a forum there that's great:
http://www.reprise.com/host/tektronix/home/default.asp
A European site for older Tek scopes that's fun is:
http://margo.student.utwente.nl/~wel/tek.htm
There's another Tek site at:
http://www.helo.de/helo/museum/tek/tekold/tekold.htm
And of course, don't forget the Tektronix corporate site where you should still be able to order the old manuals on microfiche:
http://www.tek.com
So, is that enough poop on the subect?
Good luck with your 561!
Dean