Dry Ice, as mentioned by Adrian Mole is the easiest way to go about it.
There are also cool containers available, with the IATA designator RAP. Basically 747 belly containers which are insulated, packed with Dry Ice & for some reason need a dozen or so batteries (for a fan I guess).
Finally, modern freighters like the 747-400F have a state of the art aircon system, where temps of +4C to +29C can be dialled into the system.