Do it yourself

To all, I have a water supply valve (not a sprinkler head) that is not functioning. I need to find it and repair or replace it. Trouble is the valve is buried in my yard and I have no idea where it is located. I talked to a local lawn maintenance service about this. For the low price of $65/hr they'll come out and locate it with a special "device". I asked what specifically this device was, how it worked, and where I might be able to rent one. Needless to day few details were volunteered. I tried poking around in the yard with a thin metal rod in hopes of finding the valve box, but that turned up nothing. I really want to fix this valve myself in order to save money, here are my questions: - What is the best way to locate this valve? - Does anyone have any information on the device this lawn maintenance service might be referencing? - If so where might I be able to find or purchase such a device? - Typically, how deep are these supply valves buried? A couple of facts about my sprinkler system: - There are 4 zones (zone 4 doesn't work, all the rest do) - The wiring from the controller to the water supply valves appears to be run in a single wire bundle... which leads me to think that the 4 water supply valves are located close to one another. Any help or advice would be deeply appreciated.