I generally use Google Flights and see all options from there. It gets the job done. Prices tend to be better on Tuesdays and Wednesdays, although take that as a general guideline.

Other platforms I've used in the past are CheapOAir and Skyscanner. For more spontaneous trips, Travel Pirates and Going (formerly known as Scott's Cheap Flights) have great deals. Those may also be worth checking out and searching for any ongoing trips to the US.

Where in the US are going? That will also influence the price. One possibility is traveling to a less traveled to airport and driving or commuting (bus, train, etc.) to your destination, depending on where you go. That said, with the cost of a rental car or rideshare you might end up paying the same anyway.