What are some places I can find cheap airline tickets to Europe to be specific?

I want to take a trip to Warsaw in August for 10 days. I was wondering if anyone knows any cheap airline websites or some kind of coupons since tickets to Europe are over a grand per person. To be exact, I'm looking for a website that offers children to be 18 and under (for a 16-year-old). This flight will be for 3 adults and one child.