Steeda, Flex-a-lite, and B&M all make good quality transmission coolers. The bigger, the better in most cases. Be sure which ever one you get has a fin design and if you plan on driving your car hard or racing frequently, a transmission cooler with a built in fan unit for extra cooling would probably be a good idea.
On a side note, be sure to keep your transmission fluid and filter changed frequently for maximum protection and performance. This is critical in a high performance automatic transmission.