HAG Office Chair - I had bad back problems from practicing in front of computer on a bad chair. This one cost a fortune but it has back support and retractable arms http://www.hag-uk.co.uk/products/hag-h05/ may not be available in US though
I use the office chairs (without arm rests...)... easy to find at any office supply store...
They have wheels too.... you can roll over to the table to grab your coffee or other assorted beverage of choice... swivel back and forth... or just get straight up whacky while sitting and playing...
I LOVE them.... makes Guit-Fiddlin' even BETTER than it already is
I use a 24" wooden stool. I bought it from the Maine State Prison showroom- they have been making them for many years, best quality I've seen. They don't look that comfortable, but they are.
Here's a picture from their showroom:
Like others here, I use an armless office chair and a foot rest. I also have a back support pillow that I use as well. I primarily play and practice sitting down so a comfortable chair is important for me.