Can't help you much, except to say to try x-posting this in woodworking.

Find a saw mill if theres a local one. Or you could try going to a big box store and ask them to order what you want. But big sheets of solid stock is going to be expensive too.

Are each tread going to be rectangular in shspe or does each tread vary in shape?

The other idea would be is to buy a large sheet of solid 1/4" oak (4x8) from home depot or what ever is by you and then laminate that to plywood. It wouldnt be perfect in look but thats probably the "cheapest"

You do not want solid oak in that size; it will move too much. A tread made of narrower boards glued together will be more stable and less apt to bow, cup or curl. I would think that it would not cost all that much compared with solid 15" wide oak...even if you were to be able to find such a thing.

We have found oak on top, pressed wood in middle and oak bottom in sheets that are wide and are 3/4 inch thick. We would have to use a flex wood that you can sand and finish for around the step. Real inexpensive.
The spiral steps are now a plywood dark stain not finished around the edges hated the look so much completely carpeted them. Now going with ash 4" prefinshed hardwood floors and want a nice look on the steps