Silvia,
You should try floridapets.com I bought 50 from them in January, they are great. They hide in the sand and come out as soon as food hits the water, it's pretty amazing. I think they sell 25 for like 14 bucks plus shipping. They ship 2 day, I think it cost me 7 to ship... Don't worry about the 2 days, like I said, I did it in January and not one snail died, they are really tough.