Pool heaters that can be installed in aboveground pools are available from a variety of suppliers, including Amazon.com and Wal-Mart. Heater models come in gas, electric and solar models.
Electric and gas units require some preparation, with a flat surface required for operation. Generally a level concrete pad is considered the best place. Solar units do not require a flat surface but need to be located in an area that receives significant direct sunlight. A plumbing line from the pool to the heater and back will need to be installed to circulate the heated water. If an electric or gas heater is installed, an appropriate electric or gas line will also need to be installed.