If you go to Pensacola, Water World, 2410 Langley Ave, (850) 474-8887. I purchased some Emerald Crabs my last trip down there last summer.
Also in Pensacola, PetLand, same stripmall area as Petsmart [6251 N. Davis Hwy.]. I apologize I don't have the address for Petland. If you have a few hours to spare, Petland has a wonderful setup for puppies where the family can go into a small area with a individual puppy and get acquainted. They have a variety of pets you can purchase. Although their salt water fish area is not as big as Water World. I would purchase from either of these two stores.
Make sure you tell them that you are traveling so that they can prepare any livestock you purchase. Call them in advance to see if they have what you are looking for and to make sure that they are open. Water World is closed on certain days.
I was told that there was another salt water place in another sripmall area next to a highway with an overpass. I never made it to that one yet. You can probably search the Internet for that place.
I'm not sure if the long drive to Pensacola from Destin would be worth it, but if your in the area, you may want to check them out. I hope this information is helpful.