You don't need a megger.
A megger is used to test insulation breakdown. It is a meter that puts out around 1,000 volts and checks to see if you have any ground leaks. I use 'em all the time on industrial drive motors. They will shock the piss out of someone if you clip the leads on them when they are not looking, and they light up flourescent lights pretty well.
There are buried line tracers you can buy/rent, but the cheapest way and probably the best way to go is to dig the line up and replace it. It broke once, and they usually break again and again due to moisture getting in the cable/age/etc.
(Edit)
I'm sure you've done this already, but make sure the cable really is broke, and not the lights or something. Turn off the power, try to get both ends of the buried cable (Before it goes into the lights), connect the hot and neutral together, and check the other end with a voltmeter. If you get a few ohms, its good, and the lights are at fault. Connect the hot to the ground wire and check if you have a reading. If you do, the neutral is bad, and you can hook up the neutral on the lights to the ground as a fix.
If you really want to fix it right, dig her up. Run all new direct bury Romex. I also recommend you have a certified electrician inspect your work before you bury it. Sometimes they give good hints/pointers, and save your ass from your insurance company in case you screw up and burn your house down.
