If you can get your hands on a good quality metal detector for a few hours you can do it yourself, there are some places that rent them out by the hour.
Your other option could and might well be an expensive one, there are companies out there that specialize in construction x ray, mainly for high rise and pressure sensitive building. These will not be cheap (if you can even them to come out ).Here is one place that does what your looking for
Ground Penetrating Radar Services