Can a person buy a new shirt that he will not ordinarily make a shehecheyanu on during the Nine Days, but just will not wear it until after the Nine Days?
One should refrain from buying new clothes in the Nine Days, even if they are due to be worn after the Nine Days. It is only permitted to do so when there will not be a similar opportunity to purchase the clothes after the Nine Days, or if they are currently on sale price (and won’t be later).
Rema 551:7; Mishnah Berurah 551:49. Concerning somebody who has no old shirt for Shabbos, see Biur Halachah 551:6 and Iggros Moshe, Orach Chaim III, no. 80, according to which a new shirt can be worn; it is possible that buying a new shirt will also be permitted for this purpose, though this might be more stringent, because it is compared (Mishnah Berurah) to sewing a new shirt.