Go through your results for flights to St Louis in USA with Netflights. The most popular airlines for this route are United Airlines, British Airways & Virgin Atlantic, with United Airlines 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 you don’t have set dates for when you want to fly to St Louis, try searching by month – that way, you can compare airfares throughout the year. You'll probably see that prices will be higher during September, when St Louis is very popular, while January regularly has the cheapest prices.
St Louis receives 163 direct and indirect flights every week, but the cheapest airline is normally Virgin Atlantic. So, if the flights to St Louis don't work for you, you could always try searching for flights to other airports in 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.