Yeah, sizes like that are hard to find. I found a locally owned small lumberyard/hardware store that was a great source for hard to find building materials when my wife and I were repairing/restoring a family home built in the early 1930s. They were also a great source of information for resellers of stuff they didn't carry.
Not sure if it will work for you, but Southern Yellow Pine pre-cut stair treads might be worth considering. The grain isn't the same as oak, but SYP is pretty hard, takes finishes well, and can be found in widths that you might be able to use after you trim off the bullnose.
@chargepositif .. I got a 1X12 6 foot long of Red Oak from Lowes for $85.00. Cut it down to 30 inches on glued it up. I don't like engraving a cutting board that will be used. Too many places for food to get stuck and grow bacteria.
Not sure if it will work for you, but Southern Yellow Pine pre-cut stair treads might be worth considering. The grain isn't the same as oak, but SYP is pretty hard, takes finishes well, and can be found in widths that you might be able to use after you trim off the bullnose.