With a judgment you have a right to post-judgment discovery. That can take the form of interrogatories, a request for production of documents and records, or you can depose him.
Informally, you could ask someone in a position to know (such as a supplier) where he's banking these days. You could have a friend mask their caller ID and call him up with a phony survey to ask whether he's satisfied with his current bank. Once you get the name out of him, go levy the bank. You could also have someone buy from him using a check, and then once the check clears examine the check or its image for the ID of the bank where it got deposited.
You could also attempt a "rolling levy" ... just work your way down from the banks with the largest market share in your area to those with the smallest until you hit paydirt.