How can I find a digital forensics expert to help trace stolen bitcoins?
I'm not sure of the existence of a directory of experts by physical location since Bitcoin is a digital currency.

Elliptic is a blockchain intelligence company identifying illicit activity on the Bitcoin blockchain and providing services to leading Bitcoin companies and law enforcement agencies globally. Their HQ is in London and main office in New York. More information of the company is available here. If you're based in either of these locations you could contact them locally.