Unicorn item. I was lucky that a forum member in NJ was selling one for $100 about 5 years ago and I didn't hesitate to buy it. Keep your eyes out...the better seats do become available from time to time - but the Corbins are never inexpensive. Of course, if you don't want to wait you can always spend $500+ for a new one. As an alternative, you can bring your seat to a custom builder and have them upgrade the foam to a 3-layer. Heck, you could do it yourself if you're comfortable with shaping a new seat. I did it on my last project and it turned out great.
Here's what I used. This is aircraft grade stuff, but you can find automotive grade almost anywhere. It's multi-layer open-celled urethane foam.