DanaK wrote:

I'd like to set up a set of water misters to run when the outside
temperature gets to between 90 and 95 degrees...

How about running them when the AC fan runs?

The switch would then run a solenoid valve, probably something like
the 24vac automatic sprinkler valves after applying electricity to
a 120vac to 24vac step-down transformer.

How about 2 120V solenoid valves from a dead washing machine in series?

The application for this is to provide a precooled air source to my air
conditioner's compressor-condenser coil. Since we're in a hard water
area I was figuring I'd need to provide some kind of conditioned water
for this system to prevent the scaling up of the coil...

That won't help, if the water touches the coil. I save 20% by trickling
rainwater from some tubing with holes over the coil of a window AC.
The water runs down into a plastic drum containing a $10 10 watt
Harbor Freight submersible fountain pump.

All of this does cost a little but the electrical load reduction on
temperatures from 95 to 105 degrees could be reduced by 23% and
sometimes more from what I read.

This works best with low outdoor humidity, although Yogi Gaswami saved
about 20% in central Florida by building an evaporative cooler around
an outdoor coil with standard commercial greenhouse parts.

Nick