问一下关于“envelope”的事儿

fabioyang

录取信里面说注册要交一个“a self-addressed, stamped (EURO 1.45) envelope (DIN A4)”，外办告诉我说“The stamped envelop is another necessary thing you also have to take to the enrollment. Please prepare it before you register so that you just have to hand it in when enrolling.”
请问各位，这个信封我去哪儿买呢？买哪种的呢？非常感谢！