The Mishna Berura writes that any new clothing that does not require a bracha of Shehechiyanu may be purchased and worn during the three weeks. Today this includes the vast majority of purchases. During the nine days, even these should be avoided.
An item that does require Shehechiyanu should be avoided the entire Three Weeks. However, if it is marked down significantly it may be purchased on condition that it needs alterations or is not fully paid for, so Shehechiyanu will not be recited.