Seashells are an oldy but a goody. The fantastic thing about this type of aquarium decor, you can make it a personal experience in retrieving your shells.
Or if you want something more eye-catching and already-found, go bargain hunting at the seashell markets by the sea shore, just ask for Sally. But seriously, there are so many options when it comes to shape, color, and texture.
Use dull colored shells for an understated look, or go for something more bright and punchy to catchy the eye. Based off what you choose to go for, you can use the seashells to compliment what's already in the tank or make them the focal point.
Another option, substitute gravel or pebbles for a tank floor of sea shells. Experiment!