Preheat oven to 350°. Spray a 9x13 casserole dish with nonstick spray and set aside.

Boil water in a medium sized pot and cook shells for about 12 minutes. You want them to be slightly al dente because they will cook a little more when you bake them.

When the shells are cooked, drain well and place them in the casserole dish to cool for about 5 minutes, so that you are able to handle them without burning your hands. In the meantime, use a small bowl to mix together the ricotta, sour cream, salt, and garlic powder.

Use a spoon to fill each shell with the ricotta mixture, replacing them in the casserole dish as you fill. When all the shells have filling in them, pour the jar of marinara evenly over them and top with the mozzarella.

Bake for about 30 minutes, until the sauce bubbles and the cheese is melted. Cool in the pan for about 5 minutes, then serve.