Go through your results for flights to Great Falls in the USA with Netflights. Although Delta Air Lines usually has the best prices, it's best to double check what's available for your travel dates.
If these flights are too expensive, try searching for a different time of year. So if your dates are flexible, consider looking at different times of year.
But if you're struggling to find flights to Great Falls that work for you, try looking at other airports in the USA.
For even more options, check what other airports you could fly to Great Falls from. You might be able to make a saving by going from a different airport or city.