How do I check for airplane flights between Philadelphia and Yellowstone?

I would like to take a trip to Yellowstone next year. The only airport that is close to the park is in West Yellowstone, MT. I am trying to find a list of flights back and forth and am having trouble. I will be flying out of Philadelphia. One problem is flight duration. In some places it is given as around 6 hours and in others as around 11 hours.

I realize that there are no direct flights, but that should not be a problem. Is there some place on the Web where I can just get a list of all flights for all airlines for a given date?