I'm guessing you don't already have a background in fraud investigations?
If you are looking to start out and get your foot in the door, you can find positions at larger firms that have forensic accounting and litigation support as part of their practice. There are a couple of firms that only do fraud/forensic accounting/litigation support, but they are few and far between.
If you already have experience, a lot of law firms and attorneys hire accountants that specialize in this area. You have to keep in mind that you'll have to be able to backup your expertise with knowledge and experience of investigations, fraud, business valuations, courtroom testimony, rules of evidence, and so on.
If you want to be in the trenches, you can always apply to the FBI, DEA, Secret Service, and other Federal Law Enforcement agencies. They are highly interested in CPAs for criminal investigations surrounding fraud, money laundering, etc. Just be aware that their hiring process is very long.
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