Samyang's 12mm f/2.8 AS NCS fish-eye lens appears to be a good performer for its type. It should also come with an affordable price tag when local retailers stock it.

http://www.photoreview.com.au/reviews/lenses/full-frame/samyang-12mm-f-2.8-as-ncs-fish-eye-lens

This lens is nothing short of excellent optically. The design consists of 12 elements positioned in 8 groups, with three low dispersion ED glass elements and two of the elements are aspheric. As mentioned above, this lens is a stereographic fisheye. What this means is that unlike fisheye lenses that produce round images, the entire surface of a full-frame sensor is covered, and a field of view that is 180 degrees horizontally is produced.

https://luminous-landscape.com/rokinon-samyang-12mm-f2-8-fisheye-defished/

Rokinon's 12mm f/2.8 Full Frame Fisheye is a fun lens to shoot with. Built for full-frame DSLR's and mirrorless cameras, like the Sony A7 we tested it with, this lens provides you with a very unique perspective, one that's not possible with most lenses on the market today. I found that it was very easy to use and control, with precise focus at f/2.8 being the only difficult task for me during my tests. With just one instance of aberrations in our sample photos, the lens produces excellent image quality and is capable of nice bokeh at its wide open aperture settings.

http://www.steves-digicams.com/camera-reviews/rokinon/12mm-f28-fisheye/rokinon-12mm-f28-fisheye-review.html

Rokinon 12mm f/2.8 ED AS IF NCS UMC Fisheye Lens, Various Mounts, $339

https://www.bhphotovideo.com/c/product/1089934-REG/rokinon_12m_c_12mm_f_2_8_ed_as.html