While we were looking for flights for Steph we noticed a few things. The flights are incredibly cheap if you're leaving within 24 hours. We were seeing flights for as little as $150 one way to San Francisco.
We also noticed was Expedia wouldn't let us book a flight within 5 hours of departure. We had to go to the airlines own website and book directly through them, which pushed the price up significantly. It's worth it. Steph will be home by 10 tonight and will be able to go to the vet and see Bean first thing tomorrow.
As best we can tell, there's pretty good availability, so if you can deal with a little uncertainty, wait until you travel day to book you flight. You'll save a bundle!