How to find the cheapest flight when your schedule is totally open?

I am currently in South America and i am open to taking bus or flying from Peru to Colombia but i was curious if anyone knows of a way or website that allows you to say. "Give me the cheapest flight in April from Lima to Bogota." For example. I have seen some online website say "my dates are flexible" but if it is for international flights the option is disabled. Any ideas? Thanks