If you wait until after Christmas, most of the stores have huge sales and you can get stuff marked way down. I did this last year at Anne Taylor. I wore a blazer, ankle trousers, flats, and a blouse for every interview (except one where it snowed, and I switch the flats for nice boots). Everybody else was dressed exactly the same way. Most psych departments are fairly casual, and I was much more over-dressed than the faculty and the students (which is what you want). I don't think a full suit is necessary, but this is just my experience.