T. gigas has a lifespan of well over a hundred years, and while young can double their length every two years in an ideal situation. So a 10" T. gigas is likely to be very young in giant clam years.
galleon, have you thought of contacting toptank over at RC or through his website at clamsdirect.com. He might be able to look for a clam like that when he gets his orders. Just a thought.
Aquatic Creations and/or Steve Tyree had some Hippopus sp. clams, both H. hippopus and H. porcellanus. If you're looking clams that will soon be that large and are a bit unusual to boot, you might see if they still have them.
Steve
Been chasing T. gigas around for 3+ months now. Asked Barry at Clamsdirect and Jim Norris at Tanku many many times with no success. Jim shows a 6" in stock now but I can't get him to respond to any emails when I inquire about it :evil: . Good luck.