Use a map to find the street names and then run through the location of banks in the Yellow Pages. Small businesses that make daily deposits are likely to use the closest bank. If the debtor works for a large company or state institution, is in a labor union or is a member of a large association, ask an employee or member if there is an affiliated credit union. The debtor's account may very well be with a credit union. Once you have a list of possible banks, you need to start calling.
Financial institutions are very concerned about protecting the privacy of their customers.
Even so, several techniques will often let you find out customer names. One strategy is to call the bank near closing time, when things are hectic and bank people are less likely to do much checking. Identify yourself as calling from a major stereo or appliance store, with a credit application that lists the bank's name as a reference. You want to confirm that a certain person has an account there. He put down his phone number instead of his bank account number.