If someone gets satisfaction from buying a tallis katan? Its a beautifull and expensive beged, the tzitzit are meupetz, it’s chazon ish size. You put alot of effort into it. Can you or are you obligated in shechyanu? Can it be bought on sefira depending on your answer?
If the tallis katan brings you simchah, you should recite shehechiyanu over it. Shehechiyanu is a subjective berachah, depending on the inner joy a person experiences. Although Chazal give basic outlines for it, the precise importance of a new garment and the joy it induces is subjective, and if the tzitzis bring you intense joy, you should recite shehechiyanu.
Under such circumstances, it is better not to purchase the tallis katan during the Omer period, although there is no prohibition of doing so. If one bought it, shehechiyanu should be recited.
Sources: Concerning shehechiyanu over the tallis katan, see Iggros Moshe, Orach Chaim Vol. 3, no. 80; Or Le-zion (46:60) rules that one can even recite shehechiyanu over pajamas, if they bring one joy (for most people, this would mean quite extra-ordinary pajamas!).
Concerning the shehechiyanu berachah, 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), there remains a preference not to buy a new garment that brings a person joy, though if one does, one should make the shehechiyanu blessing.