In the Turks and Caicos islands conch is a very popular food. They actually have the only conch farm in the world on Providenciales. On all of the islands it is not unusual to come acrosss a huge (and I mean huge) pile of conch shells....they are actually just garbage. Tourists like to take them home as the islanders really don't use them after the conch is taken out. Keep in mind though you really are not allowed to bring any thing like that back to the US or Canada so if you are caught with them you could be fined. Kind of a drag really.
There were some kids selling conch shells next to the pier where the ultimate snorkeling excursion left from for $3. Seemed like they were in good shape, with the conch removed. As far as I know, you can bring the shell into the US, don't think you can bring the animal itself in. (Our customs procedure was kinda screwy, so it didn't become an issue for us).