You're going to want to go for the pasta shells that say "Jumbo" on the bag or box they come in. They will look a bit on the small side when they're dry but as they cook, they'll expand a bit and become flexible. After cooking, they're very easy to work with and hold a lot of filling so you should have no problems stuffing them. You can find these Jumbo Pasta Shells in the dry pasta section of most grocery stores.