I found this site (
Camera Manual Download - FotoEZ.net) can free download almost all popular digital camera user instruction manual.
That is an excellent link to find digital camera manuals but if you are a collector of vintage cameras this will not help
All you have to do is use Google for all cameras vintage to digital. Simply be creative with the search description
For example, I have a "Kodak no.1a junior camera". If I search with this description (without quotes)in Google I get this link at the head of the list.
This is found at
Kodak Autographic Junior 1A instruction manual, user manual, PDF manual, free manuals
Now this is not a site that, depending on your honour, is free as you should make a donation of $3.00-$7.00/manual. Until I came to this thread I was unaware of the Butkus site. Paying $3.00 for a PDF for a manual of a camera that you own but cannot find seems trivial to me. But he does not have EVERY manual available for every vintage camera.
For example: while Butkus has a large number of documents relating to GRAFLEX cameras, it does not have an instruction manual for a
Graflex RB Super D and RB Series B camera
Simply use this as your search term:
instruction manual for Graflex RB Super D and RB Series B camera
You will find this website:
Instruction Manual for Graflex Cameras RB Super D, RB Series B, and Earlier Models and the complete manual.
If I cannot find something on Google, I will try other search engines such as BING, Yahoo, Baidu (Chinese search engine)
, Duckduckgo and Yandex (Russian search engine) (use google translate if your Russian or Chinese is not up to snuff) 
I may not always find free manuals but usually they are less than $20 if the are found. As a collector (or in my case an accumulator of vintage cameras) this is a trivial cost for something that would enhance my enjoyment of a vintage camera.