Memory foam, whether it is from a name brand company or not, is available in a few seats on the market today. The one that is making a big splash is the Bostrom Wide Ride II. It comes in a low profile and a standard profile version. This is a wide, cushy seat. Works well for those who's body's crave that kind of comfort. Another alternative is the Kneodler line of Chief seats (Lowrider, Chief, Grand Chief). All of these seats have a more structured ergonomic upper that can be padded with memory foam as an inexpensive upgrade option.
All prices are in USD.