I bought this one from Amazon, and I'm pretty happy with it, all and all: Poetic KeyBook Bluetooth Keyboard and Leather Book Style Case for Acer Iconia A200 / A210 10.1-Inch Tablet Black.
The caveat is that the keyboard won't stay connected to my A200. It's like one side or the other goes to sleep. I have requested a replacement, and we'll see what happens. But the case is very nice, and the ability to remove the keyboard is great, both for repositioning (typing in your lap while the tablet is on an airline tray table, for example) and for leaving behind when you don't need it. At $39, it was a great value.