Are there standard lens or rather mirror systems for doing this?
There are stereoscopic lenses that let you take a single exposure with right and left images at once. One example is the Loreo stereoscopic lens:
There are many other similar devices that are or were made for a variety of cameras.
I'm not aware of an optical attachment that lets you take the two images separately, probably because capturing the images at two different times, even if they're close together, will always be a problem if there's any movement in the scene at all. It's very hard to get even a tree to stay completely still for the second or two that you might have between shots if you were working quickly -- the wind might blow the leaves, or a cloud might move and change the lighting. Animals, including people, would be impossible.
If you're aware of the possible problems and still want to experiment, my advice would be to move the entire camera from one position to the other. You could easily build a fixture for a tripod that lets you slide the camera between two positions. Find a short piece of appropriate straight metal stock (80/20 t-slotted extrusion would be perfect) and something that slides along it to which you can attach the camera (80/20 makes a variety of linear slides that fit their extrusions). Put stops at each end to establish the two positions, and then you'll be able to quickly and easily take photos in the same direction from two positions a fixed distance apart.
