While shopping for side sleeper pillows, you will notice that they come in a variety of shapes, some of them quite unusual at first glance:
Traditional rectangle. Some side sleeper pillows have a fairly standard rectangle shape.
Rectangle with arced indent. This is your standard rectangle shape, but with a concave indent along the bottom to make room for your shoulder.
U-shape, L-shape, J-shape, or V-shape. These also make room for your shoulder.
Arm rest pillows. These can vary quite a bit in terms of shape, but all offer room for you to rest your arm (nice for former stomach sleepers).