How can we find the main line to the sprinkler system ?

We bought a home with a sprinkler system. One of the lines broke just past the main shut off. We shut the main water off to the house, which did not shut off the broken pipe, so we shut off the main at the curb. We can't find where it starts. We want to put a ball valve to shut it off. The pipes are schedule 20 and are breaking we do not use the sprinkler system in the front yard.

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