@Chad: there are plenty of LM2596-based stepdown regulators you can buy on Amazon or eBay for just a few bucks, for example:
https://www.amazon.com/DZS-Elec-Voltage-Converter-Charger/dp/B0728KXFZV/
While most of these require DC input voltage, you can add a rectifier diode (like 1N4002, 1N4004 etc, anything with at least 100V reverse voltage and 1amp forward current), and the diode will form a half-wave rectifier to convert 24VAC to DC input voltage. These boards also generally have a big input capacitor to smooth out the half-wave voltage.
OpenSprinkler uses the same type of regulator except it has the circuity built-in.
Just noticed that vrbt150 already pointed out some convenient 24VAC to 5VDC adapters you can buy, such as this:
https://www.amazon.com/SMAKN-Converter-Voltage-Supply-Waterproof/dp/B00RE6QN4U
(though as vrbt150 said you will need to solder a USB connector yourself).
