What techniques can I use to locate a guy's bank if he is the defendant in a judgement?

You are not going to get that information here

Hire a investigator who specialises in it.

Go to court and get a court order to order him to tell the court where his bank accuonts are. Have someone follow him and see where he works, and garnish the pay.

See if you can run a credit report, if there has been any new bank accuonts opened normally they run a credit check.