Go through your results for flights to Casper in the USA with Netflights. Although United Airlines usually has the best prices, it's best to double check what's available for your travel dates.
Flexible on dates? Conducting your search by month allows you to compare prices to Casper throughout the year. Accommodation prices will also be higher at this time of year.
So, if the flights to Casper don't work for you, you could always try searching for flights to other airports in the USA.
You can also broaden your search by looking at more than one departure airport. Have a look around – you might be able to save by flying from a different airport or city.