I wouldn't worry about it. Like others have said, reduce your photo period and just relax.
But you could also check with sporting goods stores, as many offer CO2 refills and don't advertise it for some reason.
For next time, as jrman said, have a backup plan. Granted buying an extra tank is not always ideal, or available for many of us money wise, but knowing where you can get CO2 refills on the weekend is a good start.