110V/208V 50 va is OK try
www.Grainger.com in your area they stock residential parts and welcome DIYrs.the primary might be multi tap,but the secondary is always 24V along with that VA rating 40 or better.might want to go with a # 3TZ69 type with a circuit breaker on the 24v side to safty that end with all that goes out into the furnace and stat and condenser.