Often times, people go about researching history and finding an investigation properly. They try to ask a question and then find the answer to that question. Like Gary said, find something that interests you! But the most vital key is really to simply read. Crack open a book and actually read the content and find something curious there. It's when you read the history that a question will come to you. Granted, I completely understand that being an IB student leaves little time for 'leisure' reading of a history book, but its the best way to get top marks.
Additionally, IB loves to see local history for a few reasons. Primarily because if you're someone in Podunk, KY talking about the effect that some mayor of Podunk had on the Union soldiers during the civil war, chances are they won't have ever heard of that before. Find something uncommon. Local history also allows you the opportunity to have access to a wider variety of resources, especially primary ones.
If you have no idea where to go, start broad and quickly pair your way down. For example, U.S. history//20th Century U.S.//Cold War//Cold War Espionage//Specific spy or similar. Once you have a significantly paired down, you just have to read. If you reach crunch time, obviously this won't work. Hopefully this helps, feel free to message me with anything!