I'm using this leather case :
www.amazon.com/gp/product/B005IUVCE0/ref=wms_ohs_product
It fits well and allows access to all the ports. The openings for the camera and sensors are perfectly aligned, which can often be an issue according to the many reviews I read on other cases for the A500. It makes the tablet look rather like a leather bound book.
The cover is held closed by pair of small magnets embedded in the cover corners. They have just the right amount of holding power.