I recently discovered a large cache of stereo Mars images located on the GigaMacro site, courtesy of Neville Thompson. I will attempt to teach everyone how to use the X3D feature.
The following links below are collections of Curiosity's SOLs. Each URL loads a page of thumbnails, and you can either view each thumbnail as one image or in stereo pairs which is in the X3D format. Once you select one of the links below, pay attention to the icons that appear on each thumbnail. The ones we will be concerned with are on the LOWER LEFT side of each thumbnail. The 2 icons have little "mountains" in them and correspond to the left and right pairs. You need to click both of them (click 1 for 2D, or both for 3D). Once you do this, near the TOP RIGHT-HAND section of the screen, an orange button will appear that says "START". Click START, and it will load the stereo pair. Before you start browsing around, pay attention to the 2 side-by-side icons that are in the middle of the bottom of the screen. You want to use the one on the RIGHT. Select "SYNC VIEW" and both images will have the ability to simultaneously scroll, zoom in/out or rotate, all while you are viewing the image in X3D.
Let me know if I missed anything or if you need further assistance getting this to work. I'm typing this by memory.. :)