If not, can one buy (and then wear) used clothing during the shloshim or, after the death of a parent, during the 12 months after the loss? This is speaking of clothing that one would say a shehechyanu for were they new ("important" clothing).
Regarding second-hand new clothing after שלושים, Reb Moshe writes (אגרות משה יו"ד חלק ד' סי' ס"א אות י"ח) that since after shloshim it is only a minhag, the issur should really only be for things that are newly made.
However, if one gets huge pleasure "שמחה גדולה" out of finding "just what I"m looking for", it should not be worn , even though it may be bought.
If the pleasure is only because he found a bargin, then it would be permitted to buy and even tailor it to size.