I've been using the Microsoft Trackball Optical for years and love it (despite having to clean my hand sweat/dirt off of the 3 support rollers regularly). I've added some strips of Velcro to hold a piece of microfiber cloth to help absorb my palm sweat, which helps a bit.
I went to buy another but had to settle for the Logitech Trackball unit that looks similar, but discovered that it doesn't have the same [solid] 'feel' as the MS Trackball. I always loved the feel of those big trackballs that the arcade games (like Asteroids) used, and I've seen them occasionally on TV (science/computer-type topics) but never knew where to buy them.
After reading through this thread and checking out the Contour/RollerMouse and the Kinesis sites I plugged in the Kinesis URL into "Similar Sites
" and discovered the Fentek Industries
There I found a large selection of 'ergo' keyboards & mice, including what looks like the big (flat) trackball unit I've been looking for and a bunch of others
Also, for those looking for left-handed units, there are many flat ambidextrous roller/trackball choices.
A lot of this 'ergo' and assistive-tech stuff is expensive, but some aren't too bad. Even a guy living on SS Disability might be able to find money in his budget at some point!