Super wide-angle recommendations: I need them!

My friend Kevin wrote on Twitter:

@frenchpressme

· Jul 27 Hi @jj_lee, know you are passionate about cameras. Looking to buy or rent classic ultra wide lens to use on my Fuji X mount system (manual with adaptor) Any ones you are looking to sell or leads? Ideally 12mm or less rectilinear. Thanks!

So late in the manual on digital camera game, are there any gems to be found out there 12mm or less that are actually affordable?

I thought M42 but I assume it is entirely tapped dry and there are no bargains to be found.

I think Kevin would be  happy to rent; but rent what?