It is forbidden for a mourner to wear new clothing. From the letter of this applies only during the week of Shiva, however the world is accustomed to be stringent throughout the 12 months of mourning. [This prohibition applies only to clothing of importance, other clothing however which are not of importance, such as pants [or a shirt, and certainly undergarments] are permitted to be worn after the Shloshim.]
In a time of need: In a time of need [that one does not have other clothing available] one is to give his new clothing to another person to wear for two to three days and he may then wear it. [This allowance applies even to clothing of importance, such as a coat or Kapata and the like. This allowance applies even during Shloshim.]
One does not wear new clothing of importance for the first 12 months. If one does not have other clothing available, one is to give the new clothing to another person to wear for two to three days and he may then wear it. Clothing that are not importance, such pants, shirts, undergarments, may be warn after Shloshim.
If one tore his jacket [Keria] may he buy and wear a new one during the first year?
Yes. One may give it to another to wear 2-3 days and then he may wear it.
If one does not have Shabbos clothing available may he wear new Shabbos clothing?
If the mourner is after Shloshim then he may wear new Shabbos clothing in such a case. [This applies even to clothing of importance, such as a Kapata.]
 Michaber 389/1; Ramban in Toras Ha’adam
 The reason: If a mourner may not wear freshly laundered clothing then it goes without saying that a mourner may not wear new clothing. [Michaber ibid]
 Rama 389/1 regarding laundering clothing; Rama 389/3 regarding making new clothing and wearing Shabbos clothing; See Teshuvah Meahava 3/413-389; Pnei Baruch 18 footnote 21
 Aruch Hashulchan 389/11; Nitei Gavriel 10/13; Pnei Baruch 18/11 in name of Rav SZ”A and Tzitz Eliezer in Ohel Chana that the allowance applies to clothing which absorb sweat.
 Beir Hagoleh 389 in name of Kol Bo; Aruch Hashulchan 389/11; Daas Kedoshim 389/5
 Aruch Hashulchan 389/11
 Beir Hagoleh 389 in name of Kol bo
 Teshuvah Meahava 3/413-389; Siach Yitzchak 476