Are you looking for plans to build or to buy them? The scoring chairs are very similar to lifeguard chairs with wheels and a scoring rest, so you might look around for something you could buy and modify. There was a company in SD (Advance Sign & Engraving) that made them but the owner retired a few years ago.
We keep this one around for when Tron stops by. Since we went over to automatic doubles machines the trap boys don't have much to do, so we have them carry it. They grumble a bit but a touch of the quirt and they're clicking their heels again.
We just bought Berlin Garden bar height chairs in the Comfo model. The place in SD that was named Advance Sign...retired and nobody apparently took over the business. I call and talked to the guy in Ohio that someone linked in a previous post, those were $1000 each form their chairs.