You should find out what agency is responsible for regulation of septic systems in your area. In Massachusetts, where I work, the town Board of Health administers the approval process for residential septic systems. They have records of system designs for each house in town, provided that the system is newer than roughly 15 years. You may have a similar resource where you live. If so, you should be able to get copies of plans, then scale off distances to measure from surface features, such as building corners.
In addition, there are tools for following underground pipes that some engineering companies have. They may be able to trace your pipes without digging for a fee. They won't be able to hit the dbox precisely, but may narrow the search down a bit. Also, a quality metal detector may pick up the reinforcing bar in a concrete dbox.