These are durable and may include built-in waterfalls. Many are quite small. Use a liner specifically designed for pools. While other plastics initially may be cheaper, many are not resistant to ultraviolet light and will break down quickly. Some plastics may also be toxic to fish.
Liners also come in different thicknesses.
A thicker liner tends to be more resistant to punctures. While expensive and requiring more expertise to install, cement is also an option as a pool liner. If you use PVC, you will need to get a liner large enough for your pool. To determine how large a piece you will need, determine the maximum width, length, and depth of your pond. Multiply the maximum depth by three.