If you're still interested in the Keystone you can try contacting Java Photo (http://www.javaphoto.com/filmpage.html) who advertise 50ft magazines loaded with B&W negative, but I don't know how up-to-date their webpage is.
I don't know too much about the S8 Elmo, other than what you could find yourself doing a google search. Like all S8 cameras it takes S8 carts that are readily available, but if you want automatic exposure you'd need to source a replacement for the original PX25 3.9V mercury battery that powers the internal light meter, and it probably won't read correctly anyway for modern film stocks above 100 ISO. You could just use it manually though, using your own light meter. Unlike the other 2 cameras it's a reflex camera, which makes focussing and framing easier.
The Bell & Howell 252 is a very basic 8mm camera that takes 25ft spools of Double 8 film, which are still available (though very limited in stock choice these days). Try John Schwind if you're in the US:
The Director's Series Bell & Howell cameras are later, better quality 8mm cameras that also take Double 8 film on spools (some models are also 'magazine' types, so watch out for that). I've got a spool model and was quite impressed with how steady the image was. If you're lucky the meter still works and you can use the camera set to automatic exposure. No batteries required, even for the motorised zoom! One of these cameras (a magazine version) was used by Zapruder to film the Kennedy assassination.
Super 8 is a far more popular medium than the older Double 8 (also called Standard 8 or Regular 8), you have much more film stock choices and the cameras are almost all reflex viewing and battery powered and easy to use. But I personally love Double 8 for a number of reasons.