Ribs for a number of classic wooden boats (adirondack guide boats in particular come to mind) were made using a natural crook. They used the crook of the root of the northern white cedar IIRC.
Anyway, they would take a slab of a few inches thick, and shape it to the curve they wanted, and then saw it into four slices that would be near identical, 2 ribs per side, bow and stern (these were double ended boats).
Anyway, that's what I would do. Make one thick siyah to the curve you want, then saw it down the midddle into two matching siyahs.
Start looking for trees that have trunks that flare out abruptly near the ground.