Best option for timber choice is a top quality semisolid flooring, with a heavy solid veneer of the timber of choice. Point the tongues forward, and glue a bullnose machined piece of solid to the front face tongue. This will give a more stable tread than a solid timber tread. Solid treads in a standard staircase stay flat , because of its method of construction into the strings, in this case it may cup if not fixed securely. Semi solid is less likely to cup and shrink or expand.
If you intend to use solid timber, then try to get a top quality quarter sawn board. That is a board that has the growth rings running from the bottom face of the board, to the top face of the board, as viewed from a cut end. Prepared treads can be ordered from any joinery, but be prepared that wide boards are not cheap. Even wide boards should be made up from 1 or 2 narrow width boards to reduce the risk of cupping.
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