Jacksonville Florida, Atlantic Coast, shell questions.?

I'm vacationing in Florida and were going to the beach a lot. I've got an aquarium at home and I'm bringing back a lot of shells to use as gravel. I would really like a large one for the center piece but I can't find any. Can anyone tell me how to find one. Do I need to dig or what? There are plenty of the little nice looking ones around and I like those but like I said I'd really like a big one. So if anyone could help at all, that's be nice.