You will have to go to a dealership to get it replaced. Also try a gas station or 2 and tell them that you lost the gas cap. Also keep a rag stuffed at the top of where you put the gas into, so that it will not slosh out of the tank when you drive. Because if you do not keep that stuffed in there until you get the new gas cap, then you will have a fire on your hands for sure.
Hope this helps.