You should be able to take it to a decent electronics supply store and find one that will do the job (not Radio Shack I maen a real electronis supply store..

Try get as mush info on the relay they are using off the net if you have to.
What is important is the coil voltage most likely 120 volts.
The amount of current the contacts have to be able to take.
And whether you need Normally Open and Normally Closed contacts or both.

I have never seen a unit that uses a special relay to run just the fan.
Usually the fan is just wired across the compressor (fan is on when the compressor is on) or using the defrost thermostat contacts (fan is on all the time except during defrost).
Unfortunately I cannot find a wiring diagram for this unit.

Sorry I could not be more helpful, perhaps someone else will jump in with a better solution