Last year I raced as a crazed jail bird and Red raced as a hillbilly. I picked up the black/white striped pants/shirt at Kmart and put my riding number on the back of the jersey in duct tape. I also taped a monster mask to the back of my helmet. Red wore overalls and a flannel shirt with a toy shotgun taped to the back of his chest protector and a cowboy hat taped to his helmet. The nice thing about the jail bird suit is that I was able to wear it over my gear and it didn't really get in the way of racing. In fact that day I raced Red's 450 in the womens class and took 2nd outta 15 (I broke my 125 the day before). That's when I decided I wanted a 250F!
If you check out the costume section at Wal-Mart, Kmart, etc... you will find a few ideas (black/white jail suit, orange jumper jail suit, astronaut, cow, spongebob, french maid, vampire, scream dude, ghost, cheerleader, fireman, witch, turtle, clown, etc).