Is one allowed to buy new tefillin during the 9 days or 3 weeks? Is one then allowed to wear that same new tefillin during the 9 days / 3 weeks?
And if the answer is yes: why?
R. Moshe Feinstein zt"l says (Igros Moshe, vol. 5 (Orach Chaim part 3), no. 80) that one may buy them even during the Nine Days, "because it is solely for the purpose of a mitzvah." (By contrast, he says that one should not buy a new tallis during this time, because it is a garment and is subject to the same laws.)