It's fairly obvious that a crystal ball produces a fisheye view when you look into it... so why not have a camera look into it? That's what this thing does: mount a little crystal ball at an appropriate distance in front of a Canon PowerShot ELPH180 (or similar camera).
Clear crystal balls are very common on eBay; a 30mm diameter one sells for about $2. Unfortunately, the one I got has a number of nicks and inclusions, and it isn't perfectly round. There is also the little problem that DoF of the PowerShot isn't sufficient to keep the entire sphere in focus. As you can see by the circular samples above, the resulting images are of very low quality; in truth, door peepholes make better improvised fisheyes. Still, this is cheap, interesting, and sort-of fun as a novelty. Enjoy!
Note: there is now [a version of this for an 80mm ball](https://www.thingiverse.com/thing:3139384).