it is very tough to get letters from someone that you have no connection at all. Therefore, you have to think about some in-direct connection. As long as these people don't know you directly, they can be independent experts.
1. Ask your boss to check with his connection. He should know someone.
2. Ask your friends whether they could get that from their boss or friends or coworkers.
3. Check your citation. Ask these people who cite your paper.
4. Think of anyone that you have met in a scientific meeting.
5. Think of anyone that have some academic contact with you previously, i.e.ask for protocol, for reprint, etc.
6. Ask the editor of the journal where your paper is published.
good luck.