https://www.newschoolers.com/ns/forums/readthread/thread_id/617338/
Theres pretty much 2 options, cheap 6mm/8.5mm fisheye (depending on the brand, same ether way) it causes lens distortion (bendy lines) or the Tameron 10-16 f2.8 costing about twice as much. There are a few lenses similar to the 10-16.
Just check out my thread, I went with the 6mm because of the price but now I want the 10-16 because it has no lens distortion.