Well, this may not be easy, but probably the easiest way to get bank information is to send the debtor a set of interrogatories. These are basically a set of questions that they MUST answer or they're in contempt of court and could be arrested. In these interrogatories, you're allowed to ask for just about anything, including revealing where bank accounts are, what the account numbers are and how much $$ is in them. You also get to ask about any other assets the person may have.
The interrogatories must be formatted and filed according to your state's laws.
Once you have that information, you get a writ from the court to levy the bank account(s).