Thanks to the years of development Leia Inc's immersity AI gives the best results  for converting 2D to 3D. It is an easy to use online service.
Sign up for an account, drop in your 2D photo, and choose L|R parallel as the format to export.

Another possibility is a Marigold based system. This will give you a Depth Map which you can use to create the stereo pair. See the Resources/Editing page for instructions.

Here is another online service, using older technology