Where can I shut off the water to a water heater if I don't have one?

I think you mean “WHERE”.

Depending on your living situation – apartment, townhouse, home – you may not have direct access to the water heater. In some apartments, there is a community water heater shared by all residents (or all residents on a floor), or you may have an individual heater but in a locked closet that is only accessed by the landlord.

So you need to find out how this is set up in your building. There is no way for us to know.

Assuming you’re talking about a detached, single-occupancy home, and not an apartment, condominium or other such structure as @elbanditoroso mentions, and if you have no isolation valves for the water heater, then you’d need to shut off the next closest upstream cold water supply, and that could very well be the “whole house” shutoff. In other words, you may need to shut off water to the entire structure in order to isolate the water heater (or other fixture or water-supplied appliance).

If you do repair or replace the water heater, then that would also be a good time to add a shutoff so that you can isolate it in the future without having to shut off the supply of water to the whole house.

At the water meter out in front of your house, or where ever the water line comes onto the property.

@kritiper that’s exactly how we have to handle it. We live in a single family house.

This is another of those questions lacking in crucial details. I’m always taken aback at how many people don’t know enough about the subject in front of them to ask a question with any chance of a viable answer. From the information given, my advice to you is to stay away from your water heater, and find someone with even rudimentary understanding of how the thing works. Instructions for the operation of the heater are required by law to be attached to the device. There should also be a valve on the inlet water line going into the heater. This is the one to close to shut off the water supply. I’m curious as to why you want to close off the supply.

I’m thinking there is a pop-off valve or something else leaking. Not good to shutoff water and not shutoff power to heater.

Agreed. The question is not clearly written. I wish the OP would show up and clarify.

Sounds like the Water Heater in your building needs repair by the owner or manager.
You do not have legal access to the water heater if it is not located inside your dwelling.
Only the Manager, Owner, or Superintendent of the building can turn it on and off, the same is true with the water in general.
Call them and let them know you are having problems.
As long as your rent is paid, they have to fix it-
or you can call the Health Department or Department of Human services for help.