Some local utility providers, organizations that handle electricity, gas, and water. I think you could send a request to them. I wouldn't rely too much on old plans if I were you. A lot can change over the years, and new pipes or cables may have been installed that aren't shown on an older diagram. It's better to use modern technologies, like the ones offered by
UCG HDD. They specialize in locating and identifying underground utilities before work begins. That's a very serious process used on large-scale construction projects. But I assume there are probably options available for personal projects as well.