Answer

There is no prohibition at all against buying clothes during the Counting of the Omer. True, some infer from the Mishnah Berurah at the beginning of section 493 that it is not proper to recite the blessing “Who has kept us alive” during the Omer, but what he writes is that if the occasion presents itself, one does recite it; from this they infer that ideally one should avoid it. In any case, since we do not recite the blessing over the purchase but over wearing the garment, I do not see any problem with the purchase as you described.