Have a Kawasaki KLR 250 enduro cooling fan not working, would it be the fan switch on the bottom of the radiator or the thermostat?  

most likely the coolant temperature sensor.

but i would test the fan to be sure it hasn't burned up.

you can do this by unplugging the fan and running two wires from the fan to the battery.
see if the fan still works, if not then you need a new fan, if then you need to check the coolant temperature sensor.
most coolant temperature sensors can be tested with either an ohm meter or by simply testing for continuity  

Do you know if this would be th esame for a Polaris 500?  Where would I find the coolant temp sensor?  

Yes I do belive it would be the same...did you already check your fan to make sure that it does work?
The cooling temp is on the motor towards the front down by where your left foot goes...it's a small round sensor with 1 wire coming out of it.(blue/white)wire color if I remember right.
 Need a new fan look at this web site below

         (cut/paste)

http://www.jaycorptech.com/default.aspx


I got a NEW fan sent to the house for $80.bucks  

Have you checked the fuses?