You can try macresq.com and nexcomp.com, and maybe powersomething had it (sorry to be so vague on that one).
The OEM CD from a 333 iMac (for OS 8.6) will work on any computer up to and including the 333 iMac. Other OEM versions will not install on other machines (for instance, my ibook 8.6 won't install on my 333 iMac).
Your best shot is finding the retail 8.5.1 CD (which will work with any machine) and then download the update to 8.6 and put that on a CD too.
And if you're buying used, holding out for a manual will give you a long, long wait.
[ 08-26-2001: Message edited by: finboy ]