You can find 24VAC relays on Digikey, Mouser, or other electronic component distributors. For example, here is my search result on digikey:
http://www.digikey.com/product-search/en?FV=fff40010%2Cfff80367%2C120000a&k=relay&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=0&pbfree=0&rohs=0&quantity=1&ptm=0&fid=0&pageSize=25
Also, note that all current versions of OpenSprinkler have a built-in relay (5V coil voltage, contact rating 120V / 2A and more recent version 240V / 3A). I would be cautious to use them to actually switch power-line devices, because the PCB traces are probably not thick enough to run a high amount of current at 110V. But they can perfectly switch low voltage (like 40V or below) devices.