I need a dress for a wedding... im 13! 10 POINTS FOR THE CUTEST!?

I need a cheap dress for a wedding. Its in October in MINNESOTA, so i need one that could be for cold/chilly or for hot because here in minnesota you never know what october could be like! lol... anyway i need a cheap dress for a 13 year old 13! it needs to be modest, and not slutty, remember im 13! Also, it needs to be something YOU YOURSELF would wear... say in front of ur grandma, hence the "not slutty" part! thanks 10 points for the cutest!