I've had for many years the Pentax Stereo Adaptor kit, which consists of a clever arrangement of mirrors in a housing that screws on to the front of a standard 50mm lens with a 49 or 52mm filter thread. The resulting stereo images appear as a pair on one 24 x 36 mm frame and once mounted in a standard 35mm slide mount are viewed in the viewer that comes as part of the kit. This is intended for shooting slides, but I have a very old 1920s/1930s stereo viewer - a very simple thing that was sold with views of exotic places around the world. I experimented with shooting on b/w negative film using the stereo attachment and then printing to the correct size for the viewer. This works very well, though as with all things stereo, you have to think about what will work well and bear in mind that there will be little stereo effect with objects at infinity and that objects too close to the camera will test the system (not to mention the eyes!) The adaptor can only really be used with a standard lens of around 50/55mm, which is a bit restrictive, but for convenience it takes some beating. Having the pair within one frame allows odd stereo exposures to be made within a conventional roll. No doubt a quick search of ePay will throw up several such adaptors for sale.
Best wishes,
Steve