Giant Clam Shell - Wanted for Children Educational Purposes (los angeles)

Hi We are looking for a giant clam shell or shells to use to educate children giant sea clams.
The larger the better.
We do not really want any new shells but something that is old, hopefully a clam that died from natural causes.
If you have such a shell, please let us know the size and what you are asking for it.
We are also looking for older Native American Indian items and a Sea Turtle or Tortoise Shell Thank you so much!