Go through your results for flights to Bismarck in the USA with Netflights. The most popular airlines for this route are Virgin Atlantic, with Virgin Atlantic claiming the top spot. Although Virgin Atlantic 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.
Bismarck receives 55 direct and indirect flights every week, but the cheapest airline is normally Virgin Atlantic. So, if the flights to Bismarck don't work for you, you could always try searching for flights to other airports in the USA.
Looking at a few different departure airports will give you even more choice. See what you can find. And remember, it's best to book your flights in advance wherever possible.