I need to know what dress to wear and where to find it! HELP PLEASE!!!?

a frat is renting out a bar for a post-homecoming game party and is only inviting a small group of people. the only guidelines they gave us girls for dressing was to "look cute, like in a dress or whatever". i am a broke college student and dont have many dresses to pick from. so where can i find a new dress that will be appropriate and thats cheap?? thank you!!!