Simple 24VAC solenoid controller for Arduino
The easy way: power the Arduino with a separate 5V USB wall wart. Then, use an Arduino relay shield for the 24VAC valves. Finally, use the 24VAC only for the valves.

You can also find 24VAC to USB 5V - search for exactly that and lots of choices come up.

The reason I recommend the USB plug approach is that the losses will be less than using the 9V-ish barrel plug. But you can can do that, too, if that's your preference.