Is the only way to track down a 401K from a old job to go back to the employer? What if the employer no longer exists?

Do you know who the plan administrator is? If you have any of the old quarterly statements they sent you, it would be prinbted on that. This would be an investment firm like Fidelity, Vanguard, Smith Barney etc. or a bank or insurance company. Alternatively - if you can get in touch with any old buddies who also worked there, perhaps they know who the administrator is.