Is a woman allowed to buy intimate apparel during the 9 days for the purpose of Leil Mitzvah and it is the night after Tisha B'av?
Answer:
This is permitted.
In general, the prohibition against buying clothing and other items in the Nine Days is deferred where there is a mitzvah involved (Rema, Orach Chaim 551:2; Mishnah Berurah 14).
Moreover, the prohibition applies to important items, and not generally to underwear (perhaps "special" underwear should not be bought, but for purposes of a mitzvah it is fine).
Best wishes.