This is the Adventure Time BMO Girls Costume Hoodie available from Hot Topic (~$43). It's a "Hoodie from Adventure Time with a BMO cosplay design." I'm going to be honest: if your costume is just a hoodie you are probably not very good at Halloween or cosplay. You know I cosplayed at a convention once but got kicked out. "What were you cosplaying as?" Convention security. I got to look through people's bags!
Keep going for some shots of the rest of the Adventure Time line, including BMO everything (including backpack with lenticular face) and an awesome My Neighbor Totoro inspired shirt. See the whole line at Hot Topic HERE because I love Hot Topic and I'm really hoping your mom will drive us to the mall after school today. Tell her there's an Auntie Ann's pretzel in it for her.
Thanks to Lurena B, who wants enough Adventure Time outfits to be able to wear a different one every day of the week. That's cool, I want to be able to do the same thing but with underwear.