William Duffield
Have you tried Tim Manney? He does not appear to be actively selling tools online right now, but does have an email address for those who would like to buy a tool.

tmanney at gmail dot com

I tried one of Tim's chair seat adzes a few years ago, and it worked well. I did not, however, use it extensively enough to hollow a whole chair seat. With these, of course, it is critical to customize the handle to your hand, wrist and elbow, and the height of your bench. I am using a hand forged 18th Century bowl adze I got from the Merchant of Ashby, so that information is not going to help you at all.