right across the street from the library downtown on 300 e is a secondhand shop that has all sorts of clothes in the back, but fair warning it's an lgbt affiliated shop(it's an awesome situation, all the proceeds go to the utah aids coalition) i get a lot of good stuff there from couches to flannels to books and the prices are always good. i usually even go there before the DI if i'm looking for something, but it's right in my neighborhood.
there are some funky consingment shops on 300 s down near the tavernacle but they have more household items usually. some do have a bunch of clothes in the basement though, and they are kind of "hip" places, so flashy onsies might be there. i wouldn't go out of my way to check them out but if you're down there already pop your head in.
have you gone to a few different DIs? i always find good shit at the one on 700 s and like 100 e or the welfare square one.