How can I find a lost 401k if the company is no longer in business?

Which brokerage firm did your old company use to administer your 401k? If you have an old statement, you can call the brokerage company and get your account information... usually, even if you change jobs, your 401k will remain with the same brokerage firm.

If your old company was incorporated through the state you live in, you might check with the State's office of the Secretary of State. That office is responsible for new business filings and they should have a record of that company's officers. That might be the place to start.