Tonight in the posture troubleshooting class at Esther's house I mentioned the office chair for $20 that I bought at Ikea. It is the Rickard chair. It has space for the butt which is a little small. It works for me and I'm a medium to tall lady. The back works perfectly to do stretchsitting.
Monica
http://www.ikea.com/us/en/catalog/products/10142693/