If your location and/or your date is flexible, there are a lot of tools that have been mentioned in posts here.
Rome@Rio does not give you all the options, and its prices are more wild guesses. Kiwi will often NOT tell you as low a price as other tools, but at least its quotes are close. Kiwi and Matrix will let you search a whole month instead of Kayak’s seven days or Matrix’s five.
Skyscanner makes a wild guess, then shows you a calendar with different guesses, and only when you pick a date, givegives you a real price (but not necessarily the cheapest).
Adioso and Momondo and Hipmunk are worth checking, but often don’t give the lowest. On and on and on. When you were posting this, did you look at the suggestions and similar questions? If not, start (but don’t post) a question on cheap/low cost flights and copy the links in all the related questions.
Do you have to fly? It’s probably cheaper than train or bus, but I wonder whether a rail pass would work for something like this.