They make line locators which have a device which screws onto a hose connection, and create a water hammer. You then use a highly sensitive microphone, to listen for the water hammer and the line is where it is loudest. I have one, but have no idea where you would rent one.
Option two, push a copper wire up the line, and use a metal locator, or a radio generator detection unit, connected to the wire to locate the line. You may be able to find the radio units at larger rentals centers.
Or, call one of the line locator companies that locate lines for the utility companies, and work with them to locate it. They have the right equipment. They have a problem with the plastic pipe. If you are willing to push a wire up it, they can easily find it.