Here's a float valve similar to the ones at wal mart.
EVAPORATIVE COOLER FLOAT VALVE - HDSupplySolutions.com
All you need to do to adjust the water level, is bend the metal arm, or change the angle, and/or raise and lower the valve itself for big adjustments.
P.S.
I believe HD supply is a nation wide pluming supply house, I got my 6" PVC tubing there.