Charlie Chaplin: All you really need for this is a dark coat, some baggy black or grey trousers, a hat, and a mustache. The dark coat and baggy pants should be easy to find at the Good Will, and maybe even the bowler. If you can't find the bowler, you can get a cheap one online. For the mustache, you can find those in some toy stores and most costume stores (or just draw it on!). For extra detail, look for a cane.
Steve Martin from "The Jerk": A more simple idea than it may sound. All you really need is a bathrobe, white socks, and some dress shoes, all of which should be easy finds (if you don't already have them). If you want to augment the costume, you can look for a wig of grey hair and maybe a chair to drag around.
Zombie bride/bridesmaid: Bridesmaid dresses are a horror unto themselves, and people seem to shed these like they were on fire after the special day. A little bit of grease paint, maybe a little hair gel for a wild look, and a tear in the dress here and there, and you will be the most frightening ghoul in the room.
Anchorman/Weatherman: One thing you can count on at the Goodwill, there will always be bafflingly bad sports jackets. Go retro and pick up that brown and blue checkered number with a nice pair of mismatched slacks, and maybe a giant belt. If you want to go the extra mile and be Ron Burgandy, grab a mustache and a wig, either online or at any costume store.