You can probably find your septic tank yourself. A plumber or other plumbing professional may not be needed in this case. You can simply probe with a length of steel rod, or get one of the detectors made for this that will locate the metal used in the manufacture of your tank. You should find where the main waste water line leaves the house, and widen your search from there.
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There are a few problems you might face, including finding nothing, or something unexpected. Especially when investigating tank location for an older home, you may not find a concrete septic tank, as the restrictions were lax or non-existent then. You could find a buried car body, or who knows what else, that some crafty homebody thought would work. You may find something that isn’t even a septic tank substitute. The Crypt Keeper may have misplaced one of his crypts, or some pirate may have buried his gold in your yard. And remember, they never found Jimmy Hoffa, did they?
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Whether you find nothing or something unexpected, your local plumbing professional can help you figure it out. They will be able to find the septic tank, and possibly explain an anomalies you’ve located. Give the plumbers a call; they can help.