The process of short selling involves borrowing the stock from your broker and then selling it on the open market. You now "owe" that stock to your broker. When you then buy the stock on the open market, you close that position and return the stock. Closing a short position is referred to as "covering your short." This is all done electronically, so it's rare that you'd have to call your broker and ask if there is any stock available to borrow.