Can i buy a new tallis katan or tallis during sefira?
Answer:
Yes, it is permitted to buy a new garment during the sefira, but it is preferable to defer it until after the sefira, unless you really need it.
This applies to the tallis; for the tallis katan, there is no problem in buying it, because it does not obligate the shehechiyanu blessing.
Sources: Leket Yosher quotes from Terumas Hadeshen that one should not make a shehechiyanu during the Omer period. However, poskim defer this from halachah (see Maamar Mordechai 493:2). Based on Mishnah Berurah (493:2), it is clear that there remains a preference not to buy a new garment, though if one does, one should make the shehechiyanu blessing.