I just bought my Transformer yesterday, along with the dock. I found the Asus sleeve for the pad, and an Asus case for the pad and dock. I've seen complaints about the sleeve, and it is thin. But it has some rigid pieces in the front that add a little protection for the screen and it will flip back to make a reading stand. The case is like a typical laptop case, but for a 10" laptop.
I didn't search much for cases after I found the OEM cases. They get the job done, though I'm sure there are plenty of others that work just as well or better.