isolating 2 zone control valves from the T-T connections on a new boiler

Just installed a new Weil-McLain CGa-5 boiler. Using two (ancient, but working with previous boiler) White-Rodgers zone valves - 24 vac. There are two red, one white, wires coming off them to attach to the boiler. I caught the repeated warning in the boiler instructions to NOT attach the 3 wire valves directly to the T-T leads. It stated I must have an "isolation relay" between the two. When I search to buy one, I am faced with telling them exactly what kind of relay I need. Can you help me to know exactly which kind to ask for? Voltage... DO/DC... or something different? My knowledge is obviously limited.
Thank you for any reply; without heat right now!