Zenza Bronica made 40mm wide angle lenses for both their 6X4.5 ETR series and their 6X6 SQ series. Using 645, 40mm is equivalent to 24mm; using 6X6, it is equivalent to a 22mm.
I have both the MC and PE versions of the Zenzanon 40mm lens for the ETR series. Both perform very well, and are not expensive, especially the older MC version. ETR series bodies are likewise very inexpensive. I bought an ETRSi several years ago just to see if I liked medium format while not spending much, and have found the Bronicas and their lenses much to my liking. Simple and capable, and these days, not expensive. The other wide angles were 50mm (very inexpensive now), and the 60mm. And a fisheye, 35mm I think. The 75mm E-II or PE normal lens is also very inexpensive.
The one option wider than the 40mm is the Pentax 645 with 35mm lens-- equivalent to 21mm. A considerably more expensive setup, but with the older non autofocus lens, still not too bad. There is also a Pentax 33-55 zoom, which would give about a 20mm equivalent. They're kind of hard to find, and not cheap when you do, but FYI.