Ultra-wide lenses typically didn't exist in film days. Anything below 24mm is very hard to find. You might be able to grab an 18mm (like, say, the Olympus OM 18/3.5), but understand these were super-exotic at the time, and are liable to go for high prices. You'd probably be better off saving up for the 16-35, getting the 28/2 with the 21mm wide converter, or maybe looking at the Samyang 14mm f/2.8.
See also: Can I use lens brand X on interchangeable lens camera brand Y?Can I use lens brand X on interchangeable lens camera brand Y?