need help choosing a float switch
 

I am adding an on-demand water pump to my rain barrel system that is used to feed water to one of my house's toilets. I have all of the parts needed to set this up, but I need a float switch to prevent the pump from running in the case that we run out of water in the barrels.
The pump is a 12v ShurFlo RV pump. Is there a cheap float switch that will only connect the circuit when there is sufficient water level available?
Thanks for any help.
-M@

Try omega engineering, possibliy the lvn-90 series.