Cooling outfits are used to reduce the idle heat of a ship, making it harder to overheat. Human cooling outfits, while not the best in the game, are usually just enough to keep all but the hottest ships cool enough as to rarely if ever overheat.
Cooling ducts are cheap and are found in almost all outfitters. Water coolant systems are slightly less common, but still common enough that they can be found being sold on most planets. Liquid nitrogen coolers are sold widly in the South and the Rim, as well as in a few places in the near Earth region. Liquid helium coolers, being the best, are only sold in four places in the South and Rim, one of which is a pirate planet.