Go through your results for flights to Mammoth Lakes in the USA with Netflights. Although British Airways 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.
So, if the flights to Mammoth Lakes 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.