Can one shop for clothing and housewares for a kallah if the time constraints before the chasuna demands it?

Shalom, Thank you for your question. First of all, a big Mazel Tov to the couple. May they merit to build a house of Torah in Am Israel. You are correct that we try to limit purchases, and certainly major purchases during these days. However, if there are time constraints, and a real concern that if these purchases are not made during these days then thing will not be ready in time, it is permitted to buy all that is needed (see the Rema in the Shulchan Aruch 551,2 that for the needs of a mitzvah all purchases are permitted). One should put the new items aside after buying them, to enjoy after Tisha B'Av. Blessings.