Can I buy a 24v transformer with a dedicated common c-wire feed?

I have an oil furnace that feeds hot water to baseboard in four zones in my house and also to a hot water tank for tap water. There is a 24vac transformer with two output wires that feed power to the electric zone valves. I want to change one thermostat in a bedroom to a Wifi unit that needs a c-wire. I have read that I can connect the c-wire to the transformer (power stealing), but that is not recommended. I have 5 wires running to the old thermostat, but only Rh and W are connected. I think a good option would be to just replace the old transformer with a modern one. Why is it that I cannot find an HVAC transformer that has a dedicated common terminal? To me this makes more sense then adding a second transformer. Any recommendations?