Before asking your wife this, pause a minute and think about anything else that goes into Halloween at your house, and figure out who did it or is going to do it.
Who buys Halloween candy, decorates the house, takes the kids tick-or-treating, hands out candy at home, provides treats for classroom parties, serves a special Halloween dinner, drives kids to parties, etc.?
If all of this is divided evenly between the two of you, or you do more of it, then I think it's fair to ask why she can't get the costume.
If you realize that she does all of this, then I'd make time to get the costume.
For many women, the unfair division of the emotional and physical labor of the holidays is a big issue, even in otherwise good marriages. No need to explode that mine accidentally.
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16
