We always assemble ours from odds and ends from what we have. One year, when I was in high school, my great-grandma and I made a renaissance styled dress for me. We made it out of corduroy (because it was less expensive) and oh my was it time consuming. Turned out quiet nice, but very time consuming.
Last year at a post-Halloween sale we bought these "naughty" nurse costumes. They were actually black and kind of satin like, vs. the white pleather I always see around. So we're going as dead nurses this year. The back story (because we always create one), is that we worked in an insane asylum and were brutally killed by an escaped mental patient, now on Halloween we come back to wreck havoc and "search" for the patient who killed us. This made better by the fact that one of our friends is going as an escaped mental patient (he's wearing a fake straight jacket and pj pants) so he'll be the one we're "searching" for.
My favorite year we dressed up with my BFF as the witches from Hocus-Pocus.