I have been looking for about a year. In that time I have seen a few(less than 10) for sale. All of them were fairly large already(8"+). That doesn't help me because I want to grow mine out from around 3".
When I find one and it ends up getting too big for my tank, which will be a 225 in a few months, I will probably donate it to a zoo or a large public aquarium.
Gary, I totally understand why you ask that question. I don't think most people realize these clams can get up to 54". That is just HUGE. It would be very difficult to keep a full grown gigas at the hobbiest level. The Ca/Alk and nutrient requirements would be tremendous for a clam that size. It would also require a very large tank with thick sealed top so it doesn't literally blow the top off when "coughs".