Here's a nice deal if you have the E-mount 16mm f/2.8. The Sony VCL-ECF1 fisheye converter for the 16mm provides a 180° field of view. The VCL-ECf1's regular price is $150, it is available from Amazon at $110.
The picture below by CrepeFace shows the VCL-ECF1 attached to the 16mm lens next to the 18-55mm OSS lens.
16mm with VCL-ECF1 vs 18-55mm OSS Lens