if you have any kind of baking sheet or flat thing that you can put in the oven you can make a pan of aluminum foil on it. make sure you put several layers on the side to distribute the heat. the edges may get a little dry. you can also use a cast iron skillet, grease and flower it really Well. a 10-12 in skillet will take a mix that fits a 9x9 or 9x13. you will probably have to add time to whatever the box says for the skillet. once it's done flip it and bang it upside down on a cutting board to get the Brownies out because the hold skillet will keep cooking them even after you take it from the oven.
You should buy one or do you think it won't give you enough time to make it after you by one? You can use some sort of pan which would be VERY DIFFICULT to bake brownies with. So, I would go with buying a new one. :)