I use this one: http://www.autogeek.net/carpro-wool-wash-mitt.html
Regular wool mitts work ok but they get heavy with water when you use it and they fall apart over time because the wool falls off the hide that it is attached to. Regular wool mitts only last me a couple of months before the wool falls out and I need to replace it.
This mitt is 100% Merino wool with a super plush nap compared to regular wool. The wool is sewn into the synthetic backing so it does not fall apart. It has a mesh backing that you can stick your hand in so your hand feels comfortable and makes the mitt not as heavy when soaked like a regular wool mitt. A bit pricey but it is the best wool mitt I have ever used.